mfd: ti_am335x_tscadc: Add a boolean to clarify the presence of a touchscreen

Just checking the number of wires will soon not be enough, add a boolean
to indicate the actual use or not of the touchscreen.

Certain checks only make sense when there is a touchscreen wired. Make
these checks explicitly depend on the presence of the touchscreen.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-35-miquel.raynal@bootlin.com