ar71xx: add support for TP-Link TL-WA7510N

ar71xx: add support for TP-Link TL-WA7510N

It is mostly the same as TL-WA901N. WLAN and LAN
are working. LAN LED is working. WLAN signal
strength LEDs are not working yet.

http://wiki.openwrt.org/toh/tp-link/tl-wa7510n?s[]=wa7510n

[juhosg:
  - cleanup commit message
  - move non kernel related stuff into separate changes
  - rename and refresh 615-MIPS-ath79-TL-WA7510N-v1-support patch
  - merge WA7510N support into the mach-tl-wa901nd.c file
  - add 3.6 support]

Signed-off-by: Stefan Helmert <helst_listen@aol.de>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
SVN-Revision: 34187