ARM: AM57xx: AM43xx: Fix USB host

CONFIG_USB_XHCI_OMAP can be set for host mode without setting
CONFIG_USB_DWC3 which is meant for gadget mode only.
board_usb_init() was not being defined for CONFIG_USB_XHCI_OMAP
resulting in a data abort on usb start.

Define board_usb_init() for CONFIG_USB_XHCI_OMAP case. Move
gadget specific handling to within CONFIG_USB_DWC3.

Fixes: 6f1af1e3 ("board: ti: invoke clock API to enable and disable clocks")
Signed-off-by: Roger Quadros <rogerq@ti.com>