mtd: autcpu12-nvram: Convert driver to platform_device

Because we can have a single kernel to support multiple machines, we
need to make loading specific drivers for the target platform only.
For this, driver is converted to the platform driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>