ARM: hw_breakpoint: fix monitor mode detection with v7.1

Detecting whether halting debug is enabled is no longer possible via
the DBGDSCR in v7.1, returning an UNKNOWN value for the HDBGen bit via
CP14 when the OS lock is clear.

This patch removes the halting mode check and ensures that accesses to
the internal and external views of the DBGDSCR are serialised with an
instruction barrier.

Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>