drivers: input: gt911: enable fallback to alternate address

GT911 IC uses the INT pin to select the correct I2C address during
reset. However, some boards may not route this pin (or may only support
receiving inputs on it). This results in the I2C address selected by the
GT911 IC being arbitrary based on the state of the (floating) INT pin.

To resolve this, introduce an `alt-addr` property for this device. When
set, the INT pin will not be pulled low. Instead, the I2C address will be
probed at runtime, starting with the devicetree address and falling back to
`alt-addr`.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>