ARM: dts: imx: ventana: add LTC3676 PMIC support

All of the Gateworks Ventana boards based on the IMX6 SoC except for the
GW54xx use the LTC3676 PMIC. Add a device-tree node with interrupt support
for this PMIC. Additionally remove the simple-bus notation in the regulator
nodes and any fixed regulators that are provided by the PMIC.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>