mfd: ocelot: add external ocelot switch control

Utilize the existing ocelot MFD interface to add switch functionality to
the Microsemi VSC7512 chip.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Acked-for-MFD-by: Lee Jones <lee@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # regression
Signed-off-by: Jakub Kicinski <kuba@kernel.org>