serial: mfd: Add sysrq support

When using MFD HSU based console, sometime we need the sysrq function
to help debugging kernel. The sysrq code is basically there, this
patch just simply enable it.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>