imx: bx50v3: fix Maintainers

This fixes the warnings:

WARNING: no status info for 'ge_bx50v3'
WARNING: no maintainers for 'ge_bx50v3
Signed-off-by: NStefano Babic <sbabic@denx.de>

board/ge/bx50v3/MAINTAINERS

@@ -3,6 +3,7 @@ M:	Martin Donnelly <martin.donnelly@ge.com>
 S:	Maintained
 F:	board/ge/bx50v3/
 F:	include/configs/ge_bx50v3.h
+F:	configs/ge_bx50v3_defconfig
 F:	configs/ge_b450v3_defconfig
 F:	configs/ge_b650v3_defconfig
 F:	configs/ge_b850v3_defconfig