dt-bindings: vendor-prefixes: Add vendor prefix for M5Stack

M5Stack make various modules for STEM, Makers, IoT.
Their UnitV2 is based on a SigmaStar SSD202D SoC which
we already have some minimal support for so add a
prefix in preparation for UnitV2 board support.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://m5stack.com/
Link: https://lore.kernel.org/r/20210417011015.2105280-2-daniel@0x0f.com

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