arm, at91: support for sam9260 based smartweb board

add support for the at91sam9260 based board smartweb from
siemens. SPL is used without serial support, as this
SoC has only 4k sram for running SPL. Here a U-Boot
bootlog:

RomBOOT
>

U-Boot 2015.07-rc2-00109-g4ae828c (Jun 15 2015 - 09:31:16 +0200)

CPU: AT91SAM9260
Crystal frequency:   18.432 MHz
CPU clock        :  198.656 MHz
Master clock     :   99.328 MHz
       Watchdog enabled
DRAM:  64 MiB
WARNING: Caches not enabled
NAND:  256 MiB
In:    serial
Out:   serial
Err:   serial
Net:   macb0
Hit any key to stop autoboot:  0
U-Boot>

Signed-off-by: Heiko Schocher <hs@denx.de>