hw_breakpoints: Allow arch-specific cleanup before breakpoint unregistration

Certain architectures (such as PowerPC) have a need to clean up data
structures before a breakpoint is unregistered.  This introduces an
arch-specific hook in release_bp_slot() along with a weak definition
in the form of a stub function.

Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>