power: pmic: Consistently depend on DM_PMIC

power: pmic: Consistently depend on DM_PMIC

Kconfig symbols for two PMIC drivers (PMIC_AS3722 and DM_PMIC_MC34708)
were missing a dependency on DM_PMIC. To fix this inconsistency, and to
keep it from happening again, wrap the driver section with "if DM_PMIC"
instead of using a "depends on DM_PMIC" clause for each driver.

Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>