Input: synaptics-rmi4 - add SPI transport driver

Add the transport driver for devices using RMI4 over SPI.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>