Declare m2 clock in r8a7793 device tree.

Based on similar work for the r8a7791 by Laurent Pinchart.
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>