ARM: 6864/1: hw_breakpoint: clear DBGVCR out of reset

The DBGVCR, used for configuring vector catch debug events, is UNKNOWN
out of reset on ARMv7. When enabling monitor mode, this must be zeroed
to avoid UNPREDICTABLE behaviour.

This patch adds the zeroing code to the debug reset path.

Cc: stable <stable@kernel.org>
Reported-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>