ARM: ux500: move soc_id driver to drivers/soc

ARM: ux500: move soc_id driver to drivers/soc

As the ux500 id code is basically a standalone driver, we can move it
out of the arch code into drivers/soc/ux500.

This is a user-visible change, as it moves all the devices in sysfs
from /sys/devices/soc0/ to /sys/devices/ and leaves the soc0 node as a
separate device.

Originally the idea was to put all on-chip devices under the soc node,
and ux500 was the first platform to have this device, but later platforms
almost all didn't follow that pattern, so this makes the platform do
the same thing as everyone else.

Since the platform is really obsolete now, I am optimistic that nothing
will break after moving the devices around.

As the SoC driver no longer has access to the private header files,
I'm changing the code to instead look up the address of the backupram
from devicetree, which is a good idea anyway.

Finally, having a separate Kconfig symbol means the driver is now
optional and could even be a loadable module rather than always being
built-in if we allowed that for soc_device.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[Fixup mising Makefile, fixup BB_UID_BASE to fc0]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>