ARM: mvebu: add Armada 38x compatible string to pmsu

Since the Armada 38x PMSU registers are slightly different than the
Armada 370/XP PMSU ones, we introduce a new compatible string
"armada-380-pmsu" in the PMSU driver. These differences are not
visible for the current usage of the PMSU, but they might become
visible in the future.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1397483648-26611-8-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>