regulator: max8973: add support for MAX77621

Maxim MAX77621 device is high-efficiency, three-phase,
DC-DC step-down switching regulator delivers peak
output currents up to 16A. This device is extension of
MAX8973 and compatible with the register definition.

The MAX77621 has the SHUTDOWN pin which is EN pin on the
MAX8973. On MAX77621, the SHUTDOWN pin (active low) reset
device register to its POR/OTP value. The voltage output
is enabled when SHUTDONW pin is HIGH and EN bit on VOUT
register is HIGH.

For MAX8973, VOUT is enabled when EN bit or EN pin is high.

Add support of the MAX77621 device on max8973 regulator driver
with following changes:
- Make sure SHUTDOWN pin is set HIGH through GPIO calls if
  GPIO from AP connected to SHUTDOWN pin provided.
- Enable/disable the rail through register access only.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>