cortex_m, hla_target: do not try asserting SRST if it's not present

This should cover all the cases when RESET_SRST_NO_GATING is set
without RESET_HAS_SRST. This might happen when RESET_SRST_NO_GATING is
automatically set by a target code (and not from tcl).

However, there're some other places (mips_m4k, arm7_9_common) where
adding RESET_SRST_PULLS_TRST would lead to trying to use SRST even if
it's not present. Currently it's impossible for the user to enable
that flag without enabling SRST.

Change-Id: Ib1c6f68feed0b8057d55afd5f260bb22ab332ced
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1405
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>