ARM: at91/rtc-at91sam9: each SoC can select the RTT device to use

For the RTT as RTC driver rtc-at91sam9, the platform_device structure
is filled during SoC initialization. This will allow to convert this
RTC driver as a standard platform driver.

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