MIPS: AR7: remove platform

MIPS: AR7: remove platform

After a discussion about removing VLYNQ support from the Kernel, it was
concluded that its only user, the AR7 platform can go [1]. Even OpenWRT
has removed support because these devices are "stuck with 3.18" [2].

[1] https://lore.kernel.org/r/3395161f-2543-46f0-83d9-b918800305e1@gmail.com
[2] https://openwrt.org/docs/techref/targets/ar7

Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>