arm64: rockchip: clk: rk_clk_composite: Properly use the mask bits

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 add some debug printf useful for debuging clock problems

MFC after: 1 week