usb: host: xhci-omap: Remove redundant board_usb_init and board_usb_cleanup functions

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: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>