arm64: hw_breakpoint: don't clear debug registers in halt mode

Don't clear debug registers when halting debug mode is enabled.
This avoids wiping out break point from external debugger.

Change-Id: I891b0938e8b901231f70ae02c2c2c0cc1d3f5296
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>