ar71xx: support for D-LINK DIR-615 rev. E4

ar71xx: support for D-LINK DIR-615 rev. E4

This patch adds support for D-LINK DIR-615 E4 board. It's mostly based
on the existing support for DIR-600 A1, with some changes in the leds
configuration.

It's an updated version of the patch that reliably works on my hw for
about a year (it was built from trunk on Jan 2011). When I decided to
update the firmware and checked for the current support for that device,
I also found previously posted patches by Alexey Loukianov that
mentioned some stability issues. I'm not sure where could be the
difference, the patches are very similar except the wmac led pin number
- I was using 1, and those patches used 17.

[juhosg: add 3.2 support]

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29973 3c298f89-4303-0410-b956-a3cf2f4a3e73