ARM: mstar: Wire up smpctrl for SSD201/SSD202D

Add the specific compat string for the smpctrl registers to the
SSD201/SSD202D common dtsi.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20201201134330.3037007-10-daniel@0x0f.com

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