memory: add a driver for atmel ram controllers

Atmel SoCs have one or multiple RAM controllers that need one or multiple clocks
to run.
This driver handle those 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>