boards: imx8mp: partial pin control support

Add partial pin control support for the imx8mp. Since the UART driver is
not currently enabled, pin control cannot be tested on this platform.
Therefore, only the DTS definitions required to set the pinmux options
are present for this platform, and are not being applied (since
CONFIG_PINCTRL=n).

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