arm, spl, at91: add spl support for the corvus board

replaces the at91bootstrap code with SPL code.
make the spl image with:
./tools/mkimage -T atmelimage -d spl/u-boot-spl.bin spl/boot.bin

this writes the length of the spl image into the 6th
execption vector. This is needed from the ROM bootloader.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>