arch: arm64: Reenable HW breakpoint

HW breakpoint was disabled in
'commit f2a3f3ef ("arch: arm64: disable HW breakpoint")'.
However, it is required for setting breakpoints using ptrace
syscall, which is required for certain userspace applications
to work. So, reenable it.

Change-Id: Ib2a49488b92b0f77d13871d151778b3f0feab273
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>