boards: imx8mm: add partial pin control support

Add initial pin control support for the A53 core of the imx8mm. Since
the UART console driver is not currently enabled for this platform,
there is no way to test the full pin control enablement. Therefore,
CONFIG_PINCTRL is still not selected for this platform, although the
required DTS definitions and pin control headers are present.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>