ARM: at91:rtc/rtc-at91sam9: ioremap register bank

Instead of computing virtual address with AT91_VA_BASE_SYS, use the
appropriate ioremap() call on the driver "memory" resource.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>