arm: at91/spl: mpddrc: add mpddrc DDR3-SDRAM initialization

The DDR3-SDRAM initialization sequence is implemented in
accordance with the DDR3-SRAM/DDR3L-SDRAM initialization section
described in the SAMA5D2 datasheet.

Add registers and definitions of mpddrc controller, which is used
to support DDR3 devices.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>