power: pwm_regulator: remove redundant code

The regulator_enable() should be called from upper layer like
regulators_enable_boot_on(), remove it from pwm regulator driver.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[fixed up typo in commit message:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>