spi: spidev: add compatible value for LTC2488

Since spidev is no more allowed to use in DT and is really loudly warned about
it I'd like to add this compatible value.
(Geert Uytterhoeven wrote: "Add the compatible value for your device to the
spidev_dt_ids[] array in drivers/spi/spidev.c.")

Signed-off-by: Mark Brown <broonie@kernel.org>