ARM: hw_breakpoint: Debug powerdown support for self-hosted debug

This patch introduces debug powerdown support for self-hosted debug for v7
and v7.1 debug architecture for a SinglePower system, i.e. a system without a
separate core and debug power domain. On a SinglePower system the OS Lock is
lost over a powerdown.

If CONFIG_CPU_PM is set the new function pm_init() registers hw_breakpoint
with CPU PM for a system supporting OS Save and Restore.

Receiving a CPU PM EXIT notifier indicates that a single CPU has exited a low
power state. A call to reset_ctrl_regs() is hooked into the CPU PM EXIT
notifier chain. This function makes sure that the sticky power-down is clear
(only v7 debug), the OS Double Lock is clear (only v7.1 debug) and it clears
the OS Lock for v7 debug (for a system supporting OS Save and Restore) and
v7.1 debug. Furthermore, it clears any vector-catch events and all
breakpoint/watchpoint control/value registers for v7 and v7.1 debug.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
[will: removed redundant has_ossr check]
Signed-off-by: Will Deacon <will.deacon@arm.com>