At the moment, the edt-ft5x06 driver can control a single regulator
("vcc"). However, some FocalTech touch controllers have an additional
IOVCC pin that should be supplied with the digital I/O voltage.

The I/O voltage might be provided by another regulator that should also
be kept on. Otherwise, the touchscreen can randomly stop functioning if
the regulator is turned off because no other components still require it.

Document (optional) support for controlling the regulator for IOVCC
using "iovcc-supply".
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20210510193108.50178-1-stephan@gerhold.netSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>