cortex_m: deprecate soft_reset_halt

soft_reset_halt was only really intended for the older arm arch, eg. armv4/5.
Newer arch such as armv7m/mips do not need this as they have better alternatives.

Change-Id: Ifb360680b7e4bfa5365f3c79d82574bded952b45
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1442
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>