spi: ath79: set number of chipselect lines

All chipsets from AR7100 up to QCA9563 have three dedicated chipselect
lines for the integrated SPI controller. Set the number of chipselect
lines available on the controller to this value.

Signed-off-by: David Bauer <mail@david-bauer.net>
Link: https://lore.kernel.org/r/20210522074453.39299-2-mail@david-bauer.net
Signed-off-by: Mark Brown <broonie@kernel.org>