arm: at91: Add a header file for the real-time clock

Add register definitions for the AT91 RTC so that this can potentially be
used in U-Boot.

Signed-off-by: Andre Renaud <andre@designa-electronics.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>