iio: accel: kxsd9: Split out SPI transport

This moves the KXSD9 SPI transport out to its own file and Kconfig
entry, so that we will be able to add another transport method.
We export the common probe and add a local header file for the
functionality shared between the main driver and the transport
driver.

We make the SPI transport the default for the driver if SPI is
available and the KXSD9 driver was selected, so the oldconfig
upgrade path will be clear.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>