dts: arm64: db820c: add pmic pins specific dts file

This patch adds pmic specific dts which are configured specially
for db820c. One of such pin is GPIO_F on the Low Speed expansion
which has default output voltage of 2.7v. This patch fixes setup
for that pin to have an output voltage of 1.8v to comply with
96boards LS expansion specs.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>