ARM: 6656/1: hw_breakpoint: avoid UNPREDICTABLE behaviour when reading DBGDSCR

Reading baseline CP14 registers, other than DBGDIDR, when the OS Lock
is set leads to UNPREDICTABLE behaviour.

This patch ensures that we clear the OS lock before accessing anything
other than the DBGDIDR, thereby avoiding this behaviour.

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