ARM: omap2plus_defconfig: enable XHCI

AM437x devices have a DWC3 IP inside of them.

The host side implementation of DWC3 is XHCI
compliant. By enabling XHCI driver, we get
the USB host port on AM437x Starter Kit working
out of the box.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>