ar71xx: add support for QCA953x SoC

ar71xx: add support for QCA953x SoC

I don't have access to the specs, so I'm not sure about every detail, but I
haven't seen any problems with my test system, a TL-WR841N v9. It looks pretty
much like a QCA955x without PCI, a little twist in the clock calculation and
a AR9331-compatible switch.

Features not yet supported:

* EHCI (my test system doesn't have USB)
* ? (I have no idea if the QCA953x has any other features I don't know about
that aren't used by the TL-WR841N v9)

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40399 3c298f89-4303-0410-b956-a3cf2f4a3e73