Convert this driver over to use driver model. Since all x86 platforms use
it, move x86 to use driver model for SPI and SPI flash. Adjust all dependent
code and remove the old x86 spi_init() function.

Note that this does not make full use of the new PCI uclass as yet. We still
scan the bus looking for the device. It should move to finding its details
in the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>