power: Add support for LTC3676 PMIC

power: Add support for LTC3676 PMIC

The LTC3676 PMIC includes four DC/DC converters, and three 300mA
LDO Regulators (two Adjustable). The DC/DC converters are adjustable based
on a resistor devider (board-specific).

This adds support for the LTC3676 by creating a namespace unique init function
that uses the PMIC API to allocate a pmic and defines the registers.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Stefano Babic <sbabic@denx.de>