boards: nxp: imx8mp_evk: doc: Remove stray PINCTRL reference

Removes a reference to PINCTRL in a defconfig file, which has
been removed

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

boards/nxp/imx8mp_evk/doc/index.rst

@@ -277,7 +277,6 @@ board defconfig.
    @@ -12,3 +12,4 @@ CONFIG_CONSOLE=y
    CONFIG_XIP=y
    CONFIG_CODE_DDR=y
-   CONFIG_PINCTRL=y
    +CONFIG_ROMSTART_RELOCATION_ROM=y
 
 Then use the following steps to boot Zephyr kernel: