imx8mn: evk: update MAINTAINERS

Add imx8mn_evk_defconfig to be maintained
Typo fix
Signed-off-by: NPeng <peng.fan@nxp.com>

board/freescale/imx8mn_evk/MAINTAINERS

-i.MX8MM EVK BOARD
+i.MX8MN EVK BOARD
 M:	Peng Fan <peng.fan@nxp.com>
 S:	Maintained
 F:	board/freescale/imx8mn_evk/
 F:	include/configs/imx8mn_evk.h
 F:	configs/imx8mn_ddr4_evk_defconfig
+F:	configs/imx8mn_evk_defconfig