hla: enable DWT component and fix watchpoints

The makes sure the DWT component is always enabled so that watchpoints
work as expected.

This does need merging into the existing cortex_m logic, however at the
moment this is non trivial.

Change-Id: Ic6cccd1badb51f70a2ca8ea9ab6923788a94c1bf
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1122
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>