regulator: fan53880: Add initial support

This patch adds support for ON Semiconductor FAN53880 regulator.

The FAN53880 is an I2C porgrammable power management IC (PMIC)
that contains a BUCK (step-down converter), four LDOs (low dropouts)
and one BOOST (step-up converter).  It is designed for mobile power
applications.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Link: https://lore.kernel.org/r/20200702210846.31659-2-chf.fritz@googlemail.com
Signed-off-by: Mark Brown <broonie@kernel.org>