boards: arm: nrf9160_innblue21: use regulators

The board enabled power switches at init time, hardcoding switch
information in a board C file. This patch makes use of the regulator
infrastructure to describe such power switches in DT, making the board
specific C code obsolete. CONFIG_REGULATOR is only enabled if
CONFIG_SENSOR=y, as switches power sensors.

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