drivers/base: add bus for System-on-Chip devices

Traditionally, any System-on-Chip based platform creates a flat list
of platform_devices directly under /sys/devices/platform.

In order to give these some better structure, this introduces a new
bus type for soc_devices that are registered with the new
soc_device_register() function.  All devices that are on the same
chip should then be registered as child devices of the soc device.

The soc bus also exports a few standardised device attributes which
allow user space to query the specific type of soc.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>