imx_gpio: Add gpio compat string for imx8 SOCs

Several imx8* device trees have a compatible gpio controller which
identifies as working with the soc and imx35-gpio.

Add a compat string for the commonly included 'imx35-gpio', rather than
adding each to the table.

From a grep of our dts mirror this includes the following arm64 SOCs:

• imx8dxl
• imx8mm
• imx8mn
• imx8mp
• imx8mq
• imx8qm
• imx8qxp

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46532