target-arm: Check watchpoints against CPU security state

Fix a TODO in bp_wp_matches() now that we have a function for
testing whether the CPU is currently in Secure mode or not.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
