ventana: Add support for the LTC3676 PMIC

The LTC3676 PMIC is used instead of the PFUZE100 PMIC on the
GW51xx/GW52xx/GW53xx Ventana baseboards. In order to support the IMX6Q SoC
at 1GHz on those baseboards, we need to adjust the voltage scaling for the SW1
and SW3 DC/DC converters on the LTC3676 for 1225mV. Note that the scalar
values for the LTC3676 are board-specific as they relate to a resistor devider
chosen by the board design.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>