dt-bindings: arm: mstar: Add compatible for M5Stack UnitV2

Add a compatible for the M5Stack UnitV2 that is based on the
SigmaStar SSD202D (inifinity2m).

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20210417011015.2105280-3-daniel@0x0f.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>