drivers: regulator: pca9420: remove unused registers

The PCA9420 dt-bindings include file defines all device registers, but
many of them are not used (e.g. battery charger). It is also not the way
to use register definitions. Devicetree is about hardware description
after all. File will be cleaned up in subsequent commits.

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