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: Sam Shih <sam.shih@mediatek.com>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>