configs: mediatek: fix mt7623n bpir2 defconfig

This patch add CONFIG_TARGET_MT7623 into mt7623n_bpir2_defconfig
to fix the mt7623 compile error after building others mediatek target
platform
Signed-off-by: NSam Shih <sam.shih@mediatek.com>
Reviewed-by: NRyder Lee <ryder.lee@mediatek.com>

configs/mt7623n_bpir2_defconfig

 CONFIG_ARM=y
 CONFIG_SYS_THUMB_BUILD=y
 CONFIG_ARCH_MEDIATEK=y
+CONFIG_TARGET_MT7623=y
 CONFIG_SYS_TEXT_BASE=0x81e00000
 CONFIG_SYS_MALLOC_F_LEN=0x4000
 CONFIG_ENV_SIZE=0x1000