The only way this driver can be probed by MFD via its parent device.

No other reference to 'microchip,at91sam9g45-usart-spi' exists in the kernel.

Fixes the following W=1 kernel build warning(s):

 drivers/spi/spi-at91-usart.c:684:34: warning: ‘at91_usart_spi_dt_ids’ defined but not used [-Wunused-const-variable=]
 684 | static const struct of_device_id at91_usart_spi_dt_ids[] = {
 | ^~~~~~~~~~~~~~~~~~~~~
Suggested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Radu Pirea <radu_nicolae.pirea@upb.ro>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20200717135424.2442271-13-lee.jones@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>