soc: nrf52: Add Kconfig option for enabling REG0 DC/DC converter

Add a Kconfig option, similar to the one that is already available
for nRF5340, that allows enabling the REG0 (VDDH) DC/DC converter
in nRF52840. Make use of this option in Nordic boards: nRF52840 DK
and nRF52840 Dongle.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>