Input: add MStar MSG2638 touchscreen driver

Input: add MStar MSG2638 touchscreen driver

Add support for the msg2638 touchscreen IC from MStar.
Firmware handling, wakeup gestures and other specialties are not supported.
This driver reuses zinitix.c structure, while the checksum and irq handler
functions are based on out-of-tree driver for Alcatel Idol 3 (4.7").

Signed-off-by: Vincent Knecht <vincent.knecht@mailoo.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210305153815.126937-2-vincent.knecht@mailoo.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>