serial: sh-sci: Add device tree support for r8a7794

Simply document the new compat string.
There appears to be no need for a driver update.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
[geert: Reworded to match previous commits]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>