dt-bindings: arm: mstar: remove the binding description for mstar,pmsleep

Remove the unneeded binding description.
Compatible string is in mfd/syscon.yaml now.

Link: https://lore.kernel.org/r/20200729150748.1945589-3-daniel@0x0f.com
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: fifteenhex