ARM: mediatek: Fix description for mediatek SoCs

We support more MediaTek SoCs now, update the description.
Signed-off-by: Joe.C <yingjoe.chen@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

arch/arm/mach-mediatek/Kconfig

 config ARCH_MEDIATEK
-	bool "Mediatek MT6589 SoC" if ARCH_MULTI_V7
+	bool "Mediatek MT65xx & MT81xx SoC" if ARCH_MULTI_V7
 	select ARM_GIC
 	select MTK_TIMER
 	help
-	  Support for Mediatek Cortex-A7 Quad-Core-SoC MT6589.
+	  Support for Mediatek MT65xx & MT81xx SoCs