usb: renesas_usbhs: Revise the binding document about the dma-names

Since the DT should describe the hardware (not the driver limitation),
This patch revises the binding document about the dma-names to change
simple numbering as "ch%d" instead of "tx<n>" and "rx<n>".

Also this patch fixes the actual code of renesas_usbhs driver to handle
the new dma-names.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Felipe Balbi <balbi@ti.com>