drivers: regulator: pca9420: do not use DT_PROP_OR for booleans

DT_PROP() will evaluate to 0 when a boolean property is missing, so
there is no need to use DT_PROP_OR(..., false). Using _OR also has
side-effects, such as no errors when property is not defined in the
bindings.

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