ARM: 6768/1: hw_breakpoint: ensure debug logic is powered up on v7 cores

ARMv7 allows the debug core logic to be powered down and provides the
DBGPRSR register so that software can power-up and check the status of
the logic.

This patch ensures that the debug logic is powered up on ARMv7 cores
before we attempt to access the extended debug registers.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>