spi: remove w90x900 driver

The ARM w90x900 platform is getting removed, so this driver is obsolete.

Link: https://lore.kernel.org/r/20190809202749.742267-8-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>