arm, at91: add Siemens board taurus and axm

enable support for the siemens AT91SAM9G20 based boards taurus
and axm.

Signed-off-by: Roger Meier <r.meier@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>