ARM: mvebu: add Armada 375 support to the system-controller driver

The system controller block in the Armada 375 has different register
offsets for the system reset and other related functions. Therefore,
this commit introduces the new "armada-375-system-controller"
compatible string to identify the Armada 375 variant of the system
controller.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>