mfd: Make anatop register accessor more flexible and rename meaningfully

 - rename to anatop_read_reg and anatop_write_reg
 - anatop_read_reg directly return reg value
 - anatop_write_reg write reg with mask

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Reviewed-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>