serial: sh-sci: Add OF support

Extend the driver to with support for SCIx device tree bindings. A
minimal set of features is supported, additional properties can be added
later should the need to describe more device features arise.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>