dt-bindings: net: mscc,vsc7514-switch: add dsa binding for the vsc7512

The VSC7511, VSC7512, VSC7513 and VSC7514 all have the ability to be
controlled either internally by a memory-mapped CPU, or externally via
interfaces like SPI and PCIe. The internal CPU of the VSC7511 and 7512
don't have the resources to run Linux, so must be controlled via these
external interfaces in a DSA configuration.

Add mscc,vsc7512-switch compatible string to indicate that the chips are
being controlled externally in a DSA configuration.

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>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # regression
Signed-off-by: Jakub Kicinski <kuba@kernel.org>