arm64: debug: Rename addr parameter for non-watchpoint exception hooks

Since the 'addr' parameter contains an UNKNOWN value for non-watchpoint
debug exceptions, rename it to 'unused' for those hooks so we don't get
tempted to use it in the future.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>