[intelspi] Add SPI driver for Intel BayTrail SoC

Add SPI mode (PIO-only) support for Intel Synchronous Serial Port that
can be found in several Intel's products starting from PXA family.
Most of implementations have slight differences in behavior and in
addresses for registers subset. This driver covers only BayTrail SoC
implementation for it's the only hardware I have to test it on.

Driver attaches to ACPI bus only and does not have PCI or FDT support
for now due to lack of hardware to test it on.

"intelspi" is the best name I've managed to come up with. Linux driver
name (spi-pxa2xx) does not make sense because current implementation
does not support actual PXA2xx SoCs. And as far as I know there is no
codename assigned to Intel SSP chip.

Reviewed by: br, manu
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D8896