input: ads7846: Add missing spi_device_id strings

The SPI core logs error messages if a compatible string device
name is not also present as an spi_device_id.

No spi_device_id values are specified by the driver, therefore
we get 4 log lines every time it is loaded:
SPI driver ads7846 has no spi_device_id for ti,tsc2046
SPI driver ads7846 has no spi_device_id for ti,ads7843
SPI driver ads7846 has no spi_device_id for ti,ads7845
SPI driver ads7846 has no spi_device_id for ti,ads7873

Add the spi_device_id values for these devices.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>