ARM: at91: Add external RTC for Flexibity board

This patch enables external RTC support on AT91 Flexibity board.

Signed-off-by: Maxim Osipov <maxim.osipov@gmail.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>