ARM: at91: Add new binding for sama5d3-ddramc

The IP for the SDRAM controller found on sama5d3 SoCs is different from the g45
one. Introduce a new compatible string to reflect that.

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>