arm64: dts: imx8mp-evk: add PCA6416 gpio line names

Add gpio-line-names for the various GPIO's connected to the PCA6416
I/O expander on the imx8mp EVK.

This helps when using the new gpiod interface to find the GPIOs by name.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>