ARM: at91: fix ramc standby function registration

ARM: at91: fix ramc standby function registration

After the for_each_matching_node loop, we end up with a null value for np. Then,
of_match_node() is not matching anything and we can't register the standby
function and "ramc no standby function available" is printed.

Fix that by selecting the first available standby function. For now,
at91_pm_set_standby doesn't support multiple different standby functions and no
existing SoCs have different RAM controllers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>