Fix build warnings below, because the imx8m_usb_power is not declared

board/freescale/imx8mq_evk/imx8m_evk.c: In function ‘board_usb_init’:
board/freescale/imx8mq_evk/imx8m_evk.c:205:2: warning: implicit
declaration of function ‘imx8m_usb_power’ [-Wimplicit-function-declaration]
  imx8m_usb_power(index, true);
  ^~~~~~~~~~~~~~~

drivers/usb/host/xhci-imx8m.c: In function ‘xhci_hcd_init’:
drivers/usb/host/xhci-imx8m.c:123:3: warning: implicit declaration of
function ‘imx8m_usb_power’ [-Wimplicit-function-declaration]
   imx8m_usb_power(ctr_data[index].usb_id, false);
   ^~~~~~~~~~~~~~~

Signed-off-by: Ye Li <ye.li@nxp.com>
Acked-by: Peng Fan <peng.fan@nxp.com>