mfd/db5500-prcmu: delete DB5500 PRCMU support

mfd/db5500-prcmu: delete DB5500 PRCMU support

This platform has been obsoleted and was only available inside of
ST-Ericsson, no users of this code are left in the world. This
deletes the core U5500 support entirely in the same manner as the
obsoleted U8500 silicon was previously deleted.

Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Cc: Jonas Aberg <jonas.aberg@stericsson.com>
Cc: Per Forlin <per.forlin@stericsson.com>
Cc: Ulf Hansson <ulf.hansson@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>