ARM: rpi: enable USB keyboard

Now that the DWC2 driver supports split transactions, we can reasonably
enable support for USB keyboards. This wasn't terribly useful before
since keyboards are usually LS/FS devices, and thus require split
transaction support when attached to a USB hub such as the hub built into
the RPi model Bs.

Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>