drivers: regulator: pca9420: fix VIN current limit setting

The driver had a conceptual issue regarding current limitation. PCA9420
is able to limit the current flowing through VIN, ie input current. This
is a global setting, not individual to each regulator. This patch
creates a new DT property: nxp,vin-ilim-microamp to specify such limit.
It is applied when the device is initialized.

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