cortex_m: remove deprecation for soft_reset_halt

cortex_m: remove deprecation for soft_reset_halt

The command "soft_reset_halt" is deprecated since mid 2013 with
the commit 146dfe32956d ("cortex_m: deprecate soft_reset_halt").
Nevertheless it is still extremely useful with multicore chips
where it allows to reset only one of the cores, option not
available through asserting the chip-wide srst.
It also get useful to handle the reset on some problematic chip,
as in http://openocd.zylin.com/5489

Replace the warning about deprecation with a more light debug
message.

Change-Id: I52de6359475ba31014ae77e596a87fe88b252177
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5514
Tested-by: jenkins
Reviewed-by: Edward Fewell <efewell@ti.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>