automatically add it as parent of any bus device if none already specified

we have now a nice output per bus

barebox@Atmel at91sam9x5-ek:/
devices:
`---- platform
     `---- atmel_usart0
          `---- cs0
     `---- atmel_usart1
          `---- cs1
     `---- mem0
          `---- 0x00000000-0x07ffffff: /dev/ram0
     `---- mem1
          `---- 0x00000000-0x00007fff: /dev/sram0
     `---- mem2
          `---- 0x00000000-0x000037bf: /dev/defaultenv
     `---- mem3
          `---- 0x00000000-0xfffffffe: /dev/mem
     `---- cs2
     `---- w1-gpio
          `---- w1_bus0
               `---- w1-2d-0
                    `---- 0x00000000-0x0000007f: /dev/ds24310
               `---- w1-2d-1
                    `---- 0x00000000-0x0000007f: /dev/ds24311
               `---- w1-23-0
                    `---- 0x00000000-0x000001ff: /dev/ds24330
     `---- at91sam9x5cm
     `---- at91sam9x5ek
     `---- at91sam9x5dm
     `---- atmel_nand0
          `---- nand0
               `---- 0x00000000-0x1fffffff: /dev/nand0
               `---- 0x00000000-0x0003ffff: /dev/at91bootstrap_raw
               `---- 0x00040000-0x0009ffff: /dev/self_raw
               `---- 0x000a0000-0x000bffff: /dev/env_raw
               `---- 0x000c0000-0x000dffff: /dev/env_raw1
               `---- 0x00000000-0x0003ffff: /dev/nand0.at91bootstrap
               `---- 0x00040000-0x0009ffff: /dev/nand0.barebox
               `---- 0x000a0000-0x000bffff: /dev/nand0.bareboxenv
               `---- 0x000c0000-0x000dffff: /dev/nand0.bareboxenv2
               `---- 0x000e0000-0x000fffff: /dev/nand0.oftree
               `---- 0x00100000-0x001fffff: /dev/nand0.free
               `---- 0x00200000-0x007fffff: /dev/nand0.kernel
               `---- 0x00800000-0x07ffffff: /dev/nand0.rootfs
               `---- 0x08000000-0x1fffffff: /dev/nand0.data
     `---- macb0
          `---- miibus0
          `---- eth0
     `---- atmel_spi0
          `---- mtd_dataflash0
               `---- dataflash0
                    `---- 0x00000000-0x0041ffff: /dev/dataflash0
     `---- atmel_mci0
          `---- mci0
               `---- 0x00000000-0x1d9bfffff: /dev/disk0
               `---- 0x00100000-0x064fffff: /dev/disk0.0
               `---- 0x06500000-0x1d9bfffff: /dev/disk0.1
     `---- i2c-gpio0
          `---- i2c0
               `---- qt10700
                    `---- cs3
`---- mdio_bus
`---- spi
`---- i2c
`---- w1_bus
`---- fs
     `---- ramfs0
     `---- devfs0
`---- net
`---- global

drivers:
atmel_usart
ramfs
devfs
fat
macb
Generic PHY
atmel_nand
mtd_dataflash
m25p80
atmel_spi
i2c-gpio
mci
atmel_mci
qt1070
w1-gpio
ds2431
ds2433
mem

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>