rockchip: sdram: Move all DT decoding to ofdata_to_platdata()

It is more correct to avoid touching the device tree in the probe() method.
Update the driver to work this way. Note that only SPL needs to fiddle with
the SDRAM registers, so decoding the platform data fully is not necessary in
U-Boot proper.

Signed-off-by: Simon Glass <sjg@chromium.org>