Some boards don't have a PMIC, so they can't easily turn their power
off. To cover those boards anyway, let's turn off as many devices and
clocks as possible, so that the power consumption is reduced. Then
halt the last core, as before.
This will later be extended with proper PMIC support for supported
boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>