ARM: at91: select ATMEL_SDRAMC when using OF

When using device tree, select the Atmel RAM controller driver to handle its
clocks.

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