ARM: at91/rtc-at91sam9: pass the GPBR to use via resources

The GPBR registers are used for storing RTC values. The GPBR registers
to use are now provided using standard resource entry. The array is
filled in SoC specific code.
rtc-at91sam9 RTT as RTC driver is modified to retrieve this information.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
[nicolas.ferre@atmel.com: rework resources assignment]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reviewed-by: Ryan Mallon <rmallon@gmail.com>