ARM: integrator: hook the AP into the SoC bus

This hooks the Integrator/AP into the SoC bus when booting from
device tree, by mapping the AP controller registers first,
then registering the SoC device, and then populating the device
tree with the SoC device as parent.

Introduce some helpers in the core to provide sysfs files
detailing the use of the SoC ID which will later be reused by
the Integrator/CP patch for the same bus grouping.

Cc: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>