at91: Add support for the AT91 slow clock controller

This is available on AT91SAM9G45. Add the peripheral address and flag
definitions.

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>