usb: host: xhci-plat: add clock support

Some platforms (such as the Armada 38x ones) can gate the clock of
their USB controller. This patch adds the support for one clock in
xhci-plat, by enabling it during probe and disabling it on remove.

To achieve this, it adds a 'struct clk *' member in xhci_hcd. While
only used for now in xhci-plat, it might be used by other drivers in
the future. Moreover, the xhci_hcd structure already holds other
members such as msix_count and msix_entries, which are MSI-X specific,
and therefore only used by xhci-pci.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>