[ARM] 4935/1: AT91CAP9: enable RTC-on-RTT in defconfig.

Update the help text for RTC_DRV_AT91SAM9 to mention that the
option apply to AT91CAP9 processors too, and enable it in the
defconfig.

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>