arm, at91: add siemens corvus board

enable support for the siemens AT91SAM9G20 based board corvus.

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