boards: arm: nxp_imx: kconfig: Do not assign promptless SOC_FAMILY_IMX

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

This symbol is enabled through being select'ed by SOC_SERIES_IMX_6X_M4,
SOC_SERIES_IMX7_M4, and SOC_SERIES_IMX_RT.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>