Input: add eGalaxTouch serial touchscreen driver

There are two EETI touchscreen drivers in the kernel (eeti_ts and
egalax_ts) but both are for I2C-connected panels. This is for a different,
serial and not multi-touch touchscreen panel. The protocol documentation is
at http://www.eeti.com.tw/pdf/Software%20Programming%20Guide_v2.0.pdf

Signed-off-by: Böszörményi Zoltán <zboszor@pr.hu>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>