mfd: Extend DB8500 PRCMU mailbox defs

We have a few more mailboxes and fixed messages in the
DB8500 PRCMU, update to match the latest specification.

Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>