ARM: at91/dt: sama5d3: define mpddr clock and ramc clocks

Define the available clock for mprddr and take both mpddr_clk and ddrck in the
ram controller driver.

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>