dt-bindings: mfd: syscon: add compatible string for mstar,msc313-pmsleep

Add a compatible string for the pmsleep register region in the
MStar MSC313 SoC.

Link: https://lore.kernel.org/r/20200729150748.1945589-2-daniel@0x0f.com


Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>