mfd: Export ab8500 chip id to sysfs

This patch adds a file into sysfs for reading out chip id.
It has been requested for modem silent reboot.

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Ludovic Barre <ludovic.barre@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>