bus: qcom: add EBI2 driver

This adds a driver for the Qualcomm External Bus Interface EBI2
found in the MSM8660 and APQ8060 SoCs (at least).

This was tested with the SMSC9112 ethernet on the APQ8060
Dragonboard sitting on top of the SLOW CS2.

Some of my understanding if very vague and based on guesses and
extrapolations: the documentation in APQ8060 Qualcomm Application
Processor User Guide 80-N7150-14 Rev. A describes select features but
does not document the register bit fields.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>