Set DBGCLAIM when self-host debug is enabled

[tingwei]

Hi Will and Mark,

In recent implementation of save/restore ARM debug registers in EL2/EL3, 
we found
it's necessary to know whether self-host debug is enabled so EL2/EL3 can 
avoid
saving/restoring debug registers but no one is using debug.

In ARM PSCI, it has one option to set DBGCLAIM[1] to 1 to indicate that 
debug is
in use by a self-host debugger. Do you think it's resonable to add this 
to Kernel?

For example, can we set DBGCLAIM[1] to 1 in enable_debug_monitors() and 
clear it
in disable_debug_monitors().

Thanks,
Tingwei

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