keystone: usb: add support of usb xhci

Add support of usb xhci. xHCI controls all USB speeds of the Host
mode, that is, the SS through the SS PHY, as well as the HS, FS, and
LS through the USB2 PHY. xHCI replaces and supersedes all previous
host HCIs (HS-only EHCI, FS/LS OHCI and UHCI), and is therefore not
backwards compatible with any of them. The USB3SS’s USB Controller is
fully compliant with xHC.

Acked-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>