regulator: tps65912: Use simple equations to get register address

regulator: tps65912: Use simple equations to get register address

The address of ctrl and sel registers can be calculated by simple equations.
This patch simplifies the implementation in tps65912_get_ctrl_register and
implements tps65912_get_sel_register to replace tps65912_get_dcdc_sel_register
and tps65912_get_ldo_sel_register.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>