ARM: shmobile: r8a7794: alt: Enable PFC DU for the VGA port

Tie in r8a7794 PFC DU support to the VGA port on the
r8a7794 ALT board.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>