at91: add defines for reset type

Signed-off-by: Roger Meier <r.meier@siemens.com>
Acked-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>