dev_add_child is a very unsafe function. If called multiple times
it allows setting the same device to different parents thus corrupting
the siblings list. This happens regularly since:

| commit c2e568d
| Author: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
| Date:   Sat Nov 3 16:11:05 2012 +0100
|
|    bus: add bus device
|
|    automatically add it as parent of any bus device if none already specified
|
|    we have now a nice output per bus

If for example a FATfs is mounted this nice output per bus often ends with:

>     `---- fat0
>     `---- 0
>          `---- 0x86f0000087020031-0x86f000410df27124: /dev/<NULL>
>          `---- sram00
>               `---- 0x00000000-0xffffffffffffffff: /dev/<NULL>
>               `---- 0x00000000-0xffffffffffffffff: /dev/<NULL>
>               unable to handle NULL pointer dereference at address 0x0000000c
> pc : [<87f08a20>]    lr : [<87f08a04>]
> sp : 86eff8c0  ip : 87f3fbde  fp : ffffffff
> r10: ffffffff  r9 : 00000000  r8 : 00000003
> r7 : 86f075b8  r6 : 00000002  r5 : ffffffec  r4 : 86f07544
> r3 : 00000000  r2 : 43f900b4  r1 : 00000020  r0 : 00000005
> Flags: Nzcv  IRQs off  FIQs off  Mode SVC_32
> [<87f08a20>] (do_devinfo_subtree+0x90/0x130) from [<87f08a90>] (do_devinfo_subtree+0x100/0x130)
>
> [<87f3e070>] (unwind_backtrace+0x0/0x90) from [<87f28514>] (panic+0x28/0x3c)
> [<87f28514>] (panic+0x28/0x3c) from [<87f3e4b8>] (do_exception+0x10/0x14)
> [<87f3e4b8>] (do_exception+0x10/0x14) from [<87f3e544>] (do_data_abort+0x2c/0x38)
> [<87f3e544>] (do_data_abort+0x2c/0x38) from [<87f3e268>] (data_abort+0x48/0x60)

This patch fixes this by adding a device to its parents children list in
register_device so that dev_add_child is no longer needed. This function
is removed from the tree. Now callers of register_device have to clearly
set the parent *before* registering a device.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: Jan Lübbe <jlu@pengutronix.de>