arm64: rockchip: clk: ARM CLK improvement

RockChip clocks register have a write mask in the upper 16 bits, if a 1
is present the corresponding bit in the lower 16 ones is set.
Use this instead of always setting the mask to 0xFFFF0000.
This avoids a read of the register.
While here set the parent after changing its freqeuncy, this reduce the time
between changing the parent and changing the divider for the arm clock.

MFC after: 1 week