ARM: mstar: Correct the compatible string for pmsleep

The compatible string for the pmsleep region has changed.
Update the MStar/Sigmastar v7 base dtsi with the new string.

Link: https://lore.kernel.org/r/20200729150748.1945589-4-daniel@0x0f.com


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