arm64: dts: imx8qxp-mek: Do not use underscore in node name

Underscores are not recommended to be used in node names, so change
the pinctrl IO expander node name.

This change also makes the pinctrl node names to follow the convention
used by other pinctrl group names.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>