power: regulator: Convert regulators_enable_boot_on/off() to regulator_post_probe

power: regulator: Convert regulators_enable_boot_on/off() to regulator_post_probe

Turn regulators_enable_boot_on() and regulators_enable_boot_off() into
empty functions. Implement matching functionality in regulator_post_probe()
instead. The regulator_post_probe() is called for all regulators after they
probe, and regulators that have regulator-always-on or regulator-boot-on DT
properties now always probe due to DM_FLAG_PROBE_AFTER_BIND being set on
such regulators in regulator_post_bind().

Finally, fold regulator_unset() functionality into regulator_autoset().

Signed-off-by: Marek Vasut <marex@denx.de>