drivers: regulator: npm1100: add initial support

While nPM1100 is to be operated in fixed configuration for some
applications, it has some degree of configuration via GPIOs. For
example, mode (auto/PWM) can be configured via MODE pin. VBUS current
can also be adjusted using ISET pin, even though there is no API yet to
limit the PMIC input current.

This patch adds a new regulator class driver for nPM1100 PMIC, so that
it can be used with the standard regulator API when needed.

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