mfd: rtsx: Add func to split u32 into register

Add helper function to write u32 to registers, if we want to put u32
value to 4 continuous register, this can help us reduce tedious work.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Lee Jones <lee.jones@linaro.org>