ARM: Factor out reusable psci_cpu_off_common

ARM: Factor out reusable psci_cpu_off_common

Move parts of sunxi's psci_cpu_off into psci_cpu_off_common, namely
cache disabling and flushing, clrex and the disabling of SMP for the
dying CPU. These steps are apparently generic for ARMv7 and will be
reused for Tegra124 support.

As the way of disabled SMP is not architectural, though commonly done
via ACLTR, the related function can be overloaded.

CC: Marc Zyngier <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Thierry Reding <[email protected]>
Tested-by: Thierry Reding <[email protected]>
Tested-by: Ian Campbell <[email protected]>
Signed-off-by: Tom Warren <[email protected]>