rockchip: clk: rk3368: support configuring the DRAM PLL (from TPL)

rockchip: clk: rk3368: support configuring the DRAM PLL (from TPL)

As part of the DRAM initialisation process (running as part of the TPL
stage) on the RK3368, we need to set up the DRAM PLL.

This implements support for configuring the PLL to for 1200, 1332 or
1600 MHz (i.e. for DDR3-1200, DDR3-1333, DDR3-1600 operating modes).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>