ARM: shmobile: defconfig: Enable SII902X

The iwg23s board comes with the SiI9022ACNU HDMI transmitter,
this patch makes sure the corresponding driver gets built.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>