ARM: shmobile: document iW-RainboW-G22M-SM SODIMM System on Module

Document the iW-RainboW-G22M-SM SODIMM System on Module device tree
bindings. It is just a placeholder for the time being, the actual
implementation is not available yet.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>