dts: bindings: add nordic,npm6001 PMIC

nPM6001 is a PMIC IC which embeds multiple functionalities inside:

- 1 Fixed LDO (1.8V/15mA)
- 1 Programmable LDO (1.8-3.3V/30mA)
- 4 Programmable BUCK converters (1.8-3.3V/200mA, 0.7-1.4V/150mA,
  1.2-1.4V/150mA, 0.5-3.3V/550mA)
- 1 GPIO controller, with 3 pins
- 1 Watchdog

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