spi-nor: Add support for Intel SPI serial flash controller

Add support for the SPI serial flash host controller found on many Intel
CPUs including Baytrail and Braswell. The SPI serial flash controller is
used to access BIOS and other platform specific information. By default the
driver exposes a single read-only MTD device but with a module parameter
"writeable=1" the MTD device can be made read-write which makes it possible
to upgrade BIOS directly from Linux.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>