board: am43xx: Add maintainer information

am43xx_evm_ethboot/usbhost_boot_defconfig entries are
missing in MAINTAINER file. Adding entries for them.

Reported-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>

board/ti/am43xx/MAINTAINERS

@@ -5,3 +5,5 @@ F:	board/ti/am43xx/
 F:	include/configs/am43xx_evm.h
 F:	configs/am43xx_evm_defconfig
 F:	configs/am43xx_evm_qspiboot_defconfig
+F:	configs/am43xx_evm_ethboot_defconfig
+F:	configs/am43xx_evm_usbhost_boot_defconfig