soc: arm: stm32: Fix invalid Kconfig PM entry in defconfig file

The defconfig.series file for the stm32f4 incorrectly redefines
the PM Kconfig in order to select two dependencies, COUNTER and
COUNTER_RTC_STM32_SUBSECONDS, instead of setting a default for
them if PM is included.

This commit fixes the error described above.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>