target/adi_v5_swd, cortex_m: properly handle more cases requiring reconnect

target/adi_v5_swd, cortex_m: properly handle more cases requiring reconnect

This brings SWD reconnection procedure in line with the ARM
documentation and changes cortex_m reset procedure to make use of it.

The motivation behind this patch is to make SAM4L "reset" and "reset
halt" properly without SRST. The complication here is that EDBG issues
an additional read of DP_RDBUFF automatically right after writing
SYSRESETREQ, that leads to a FAULT which needs to be dealt with
properly. With this patch the very first ahbap_debugport_init DAP
access will make SWD layer properly reinitialise the link before
continuing.

Runtime tested with mbed CMIS-DAP + KL25 only.

Change-Id: Ic506f9db30931dfa60860036b83f73b897975909
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2596
Tested-by: jenkins
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>