dt-bindings: mfd: ocelot: Add bindings for VSC7512

Add devicetree bindings for SPI-controlled Ocelot chips, specifically the
VSC7512.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220905162132.2943088-8-colin.foster@in-advantage.com