board_usb_init()/_cleanup() should be in board files and don't have
a place in the xhci-omap driver. Weak versions for
board_usb_init()/_cleanup() already exist in common/usb.c
(for host mode) and drivers/usb/gadget/g_dnl.c (for gadget mode).

Therefore, remove init and cleanup functions from xhci-omap and
implement them in the board files.
Signed-off-by: NFaiz Abbas <faiz_abbas@ti.com>
Reviewed-by: NMarek Vasut <marex@denx.de>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>