serial: sh-sci: Add device tree support for r8a7{778,740,3a4} and sh73a0

Simply document new compat strings.
There appears to be no need for a driver updates.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>