ARM: ux500: delete U5500 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.

The cpu_is_u5500() macros that can read out the CPU ID is left
until the next kernel cycle, this makes it possible to merge
deletion of dependent drivers without breakage.

This also has the upside of removing the mailbox driver which was
our only driver that was outside the drivers/* hiearchy, now the
machine directory only handles machines and nothing else.

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>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>