ARM: shmobile: r8a7793: Add missing serial devices to DT

Instantiate all serial devices in r8a7793 device tree
and set them as disabled by default.

Based on similar work for the r8a7791 by Laurent Pinchart.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>