ram: rockchip: add controller code for PX30

This sdram_pctl_px30.c is based on PX30 SoC, the functions are common
for controller, other SoCs with similar hardware could re-use it.

Signed-off-by: YouMin Chen <cym@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>