ARM: hw_breakpoint: enable HAVE_HW_BREAKPOINT feature flag

HAVE_HW_BREAKPOINT was disabled temporarily from commit
480a82f59712136b51ee5ca05c4f22c1738010c5. Issue not seen with the
latest kernel hence re-enable the feature and fix merge issue from
previous kernel upgrade.

Change-Id: Ia6e7e8282c848dd73108e4d951b3375bb7caa63c
Signed-off-by: Jin Hong <jinh@codeaurora.org>
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Add a missing variable definition