On Armada 7K/8K we need to explicitly enable the register clock. This
clock is optional because not all the SoCs using this IP need it but at
least for Armada 7K/8K it is actually mandatory.

The change was done at xhci-plat level and not at a xhci-mvebu.c because,
it is expected that other SoC would have this kind of constraint.

The binding documentation is updating accordingly.
Signed-off-by: NGregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>