mips_m4k: Added SMP debug support for mips architectures

mips_m4k: Added SMP debug support for mips architectures

This change adds smp debug support for mips platforms. The change
leverages the exiting gdb smp support as mentioned in the OpenOCD
documentation for using gdb in smp environemnt. Added commands
smp_on, smp_off, smp_gdb to control the smp mode. The implementation
also provides a way to send Jc packet and toggle the gdb display core
context as well.

Change-Id: I0835a5aed1844b6ebf8291582912f20695346003
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-on: http://openocd.zylin.com/937
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>