usb: dra7xx: Add support for dra7xx xhci USB host

Add the support for the dra7xx xhci usb host.
dra7xx does not contain an EHCI controller so the headers
can be removed from the board file.

The xHCI host on dra7xx is connected to a usb2 phy so need to
add support to enable those clocks.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
9 files changed