Input: resistive-adc-touch - add support for z1 and z2 channels

Input: resistive-adc-touch - add support for z1 and z2 channels

This patch adds support for the z1 and z2 channels. These are used to
calculate the applied pressure. As there is no common order of the
individual channels of a resistive touch ADC, support for
io-channel-names is added (although the DT bindings stated the driver
already supports these).

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210525054634.9134-5-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>