mfd: Print ab8500 switch off cause

Instead of just printing the register value, also output some
description of the value.

Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Reviewed-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>