mfd: ab8500-debug: Add support for the AB8540

Allow GPADC debug information to be shown when executing on an AB8540
based platform.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@stericsson.com>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>