dt-bindings: mstar: Add binding details for mstar,smpctrl

This adds a YAML description of the smpctrl node needed by the
platform code for the MStar/SigmaStar Armv7 SoCs to boot secondary cpus.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20201201134330.3037007-2-daniel@0x0f.com

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