arm64: dts: qcom: db820c: Add gpio-line-names property

Add gpio-line-names property for Dragonboard820c based on APQ8096 SoC.
There are 4 gpio-controllers present on this board, including the
APQ8096 SoC, PM8994 (GPIO and MPP) and PMI8994 (GPIO).

Lines names are derived from 96Boards CE Specification 1.0, Appendix
"Expansion Connector Signal Description". Line names for PMI8994 MPP
pins are not added due to the absence of the gpio-controller support.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>