soc: nordic: nrf52: fix used define for enabling DCDC converter

Use the correct define for checking if the DCDC converter shall be
enabled.

This resolves the opposite behavior where boards that enable the DCDC
converter uses the LDO and boards where LDO is used they enable the
DCDC.

Fixes: e189fb07 ("soc: nordic: nrf52: add support for DT-based
regulators config")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>