mfd: ab8500: Kill "reg" property from binding

The ab8500 device is a child of the prcmu device, which is a memory mapped
bus device, whose children are addressable using physical memory addresses,
not using mailboxes, so a mailbox number in the ab8500 node cannot be
parsed by DT. Nothing uses this number, since it was only introduced
as part of the failed attempt to clean up prcmu mailbox handling, and
we can simply remove it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>