MAINTAINERS: MediaTek: add MediaTek team in the entry

Add MediaTek team as designated reviewer.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 182c9c7..b443c5a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -196,6 +196,7 @@
 ARM MEDIATEK
 M:	Ryder Lee <ryder.lee@mediatek.com>
 M:	Weijie Gao <weijie.gao@mediatek.com>
+R:	GSS_MTK_Uboot_upstream <GSS_MTK_Uboot_upstream@mediatek.com>
 S:	Maintained
 F:	arch/arm/mach-mediatek/
 F:	arch/arm/include/asm/arch-mediatek/