spi: ath79: drop platform data

The ath79 platform has been converted to pure OF. The platform data is
not needed anymore because of this.

Signed-off-by: David Bauer <mail@david-bauer.net>
Link: https://lore.kernel.org/r/20210522074453.39299-1-mail@david-bauer.net
Signed-off-by: Mark Brown <broonie@kernel.org>