clocksource: sh_tmu: Document r8a7778 binding

The r8a7778 is very similar to the r8a7779, and already handled by
the current driver in the non-DT case.

Signed-off-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>