nRF52833 has POWER_MAINREGSTATUS_MAINREGSTATUS_High, but no POWER->DCDCEN0
register.
This breaks all builds on this MCU.
Fix the ifdef to fix the build.
The internal DC/DC converter is more efficient compared to the LDO
regulator. The downside of the DC/DC converter is that it requires an
external inductor to be present on the board. Enabling the DC/DC
converter is guarded with NRF5X_ENABLE_DCDC, this macro must be defined
if the DC/DC converter is to be enabled.