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/dts/Makefile b/arch/mips/dts/Makefile
index 3522e6c..e2de1da 100644
--- a/arch/mips/dts/Makefile
+++ b/arch/mips/dts/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
-dtb-$(CONFIG_ARCH_MT7620) += \
+dtb-$(CONFIG_ARCH_MTMIPS) += \
 	gardena-smart-gateway-mt7688.dtb \
 	linkit-smart-7688.dtb
 dtb-$(CONFIG_TARGET_AP121) += ap121.dtb