powerpc/qe: Implement QE Pin Multiplexing API

With this API we're able to set a QE pin to the GPIO mode or a dedicated
peripheral function.

The API relies on the fact that QE gpio controllers are registered. If
they aren't, the API won't work (gracefully though).

There is one caveat though: if anybody occupied the node->data before us,
or overwrote it, then bad things will happen. Luckily this is all in the
platform code that we fully control, so this should never happen.

I could implement more checks (for example we could create a list of
successfully registered QE controllers, and compare the node->data in the
qe_pin_request()), but this is unneeded if nobody is going to do silly
things behind our back.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>