mips: rename mach-mt7620 to mach-mtmips

Currently mach-mt7620 contains only support for mt7628. To avoid confusion,
rename mach-mt7620 to mach-mtmips, which means MediaTek MIPS platforms.
MT7620 and MT7628 should be distinguished by SOC_MT7620 and SOC_MT7628
because they do not share the same lowlevel codes.

Dependencies of four drivers are changed to SOC_MT7628 as these drivers
are only used by MT7628.

Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 029d290..af3f227 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -15,7 +15,7 @@
 machine-$(CONFIG_ARCH_BMIPS) += bmips
 machine-$(CONFIG_ARCH_JZ47XX) += jz47xx
 machine-$(CONFIG_MACH_PIC32) += pic32
-machine-$(CONFIG_ARCH_MT7620) += mt7620
+machine-$(CONFIG_ARCH_MTMIPS) += mtmips
 machine-$(CONFIG_ARCH_MSCC) += mscc
 
 machdirs := $(patsubst %,arch/mips/mach-%/,$(machine-y))