boards: swan_r5: fix regulator enabling code

The board.c file is using non-existing GPIO API attributes:
GPIO_SPEED_LOW and GPIO_MODE_ANALOG. The code compiles because we end up
including soc.h (via init.h -> kernel.h). soc.h pollutes the namespace
with STM32 HAL definitions, but they do not apply in the context of
Zephyr APIs.

Looking at the schematics, PE4 is really a pin controlling a regulator,
so this PR replaces such custom init code with a regulator turned on at
boot time.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>