usb: renesas_usbhs: add DMAEngine ID specification note

renesas_usbhs DMAEngine uses D0FIFO/D1FIFO,
but the data transfer direction should be fixed for keeping consistency.
This patch explain about it on renesas_usbhs.h

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>