ARM: AT91: pm: Factorize standby function

Detect presence of second bank. So we do not need to have on function per SoC

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>