arm64: rockchip: clk_pll: Multiple improvement
Remove the mode_val from the clock definition as it's a bit unreadable.
Use mode_shift to represent which bit control the mode in the register.
Simplify some case where we can avoid a register read before changing it.
Set the PLL back to normal mode after the PLL have stabilized.
Discussed with: mmel
MFC after: 1 week