dt-bindings: input: touchscreen: convert sis_i2c to DT schema

Convert the SiS 9200 family I2C touchscreen binding from free-form
text to DT schema format.

While at it, fix the example which referenced a non-existent
"irq-gpios" property; the driver looks up "attn" via
devm_gpiod_get_optional(), so the correct property is "attn-gpios".

Signed-off-by: Ștefan Ghețu <stefanghetu9@gmail.com>
Reviewed-by: Mika Penttilä <mpenttil@redhat.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260821131039.37525-1-stefanghetu9@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>