driver: ddramc: add driver to support the low_power DDR2-SDRAM device

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
sama5d3xek: add support to low_power DDR2-SDRAM device

[voice.shen@atmel.com: align timing to golden setting, then the LPDDR2 can work stability even though random access its contents]
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
defconfig: update boards default configuration files

Due to adding new board: sama5d3_xplained and supporting CONFIG_LPDDR2 config

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>