MIPS: Add support for Texas Instruments AR7 System-on-a-Chip

This patch adds support for the Texas Instruments AR7 System-on-a-Chip.
It supports the TNETD7100, 7200 and 7300 versions of the SoC.

Signed-off-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Eugene Konev <ejka@openwrt.org>
Signed-off-by: Nicolas Thill <nico@openwrt.org>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>