[Arm] at91sam9260ek: Add missing AT91_MASTER_CLOCK ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

[memory layout] streamline memory layout ...

Memory layout can now be specified via kconfig options. Two
possibilities exist: default layout means the layout is stack
/ malloc heap / U-Boot. The user can also specify fixed addresses
for each TEXT_BASE / stack / malloc heap.

add basic at91sam9260 support. Currently only second stage ...

bootloader is supported:
- No SDRAM initialisation
- No UART init / baudrate change