Add support for the alternate (second-source) touchpad found on some
X13s laptops.

Note that alternate touchpad is kept disabled for now. The boot firmware
should determine which device is actually populated and enable only the
corresponding node.
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220805092317.4985-8-johan+linaro@kernel.org