ARM: hw_breakpoint: ensure OS lock is clear before writing to debug registers

ARMv7 architects a system for saving and restoring the debug registers
across low-power modes. At the heart of this system is a lock register
which, when set, forbids writes to the debug registers. While locked,
writes to debug registers via the co-processor interface will result
in undefined instruction traps. Linux currently doesn't make use of
this feature because we update the debug registers on context switch
anyway, however the status of the lock is IMPLEMENTATION DEFINED on
reset.

This patch ensures that the lock is cleared during boot so that we
can write to the debug registers safely.

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