ARM: hw_breakpoint: check if monitor mode is enabled during validation

Rather than attempt to enable monitor mode explicitly when scheduling in
a breakpoint event (which could raise an undefined exception trap when
accessing DBGDSCRext), instead check that DBGDSCRint.MDBGen is set
during event validation and report an error to the caller if not.

Signed-off-by: Will Deacon <will.deacon@arm.com>