The ili251x firmware protocol permits readout of panel resolution,
implement this, but make it possible to override this value using
DT bindings. This way, older DTs which contain touchscreen-size-x
and touchscreen-size-y properties will behave just like before and
new DTs may avoid specifying these for ILI251x.

Note that the command format is different on other controllers, so
this functionality is isolated to ILI251x.
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20210831202506.181927-1-marex@denx.deSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>