dts: bindings: regulator: add nordic,nrf5[2]x-regulator[-hv]

Some Nordic SoCs, like nRF52 contain an internal regulator for the main
SoC supply. Depending on the SoC, the regulator can have multiple
configurations (e.g. single/double stage), which mainly depends on
supporting "high-voltage" mode or not (VDDH pin supply). This patch adds
bindings for nRF5X regulator and nRF52X HV regulator.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>