rockchip: rk3128: add sdram driver

rockchip: rk3128: add sdram driver

RK3128 support up to 2GB DDR3 sdram, one channel, 32bit data width.

This patch is only used for U-Boot, but not for SPL which will
comes later, maybe after we merge all the common code into a common
file.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>