rtc: remove w90x900/nuc900 driver

The ARM w90x900 platform is getting removed, so this driver is obsolete.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190809202749.742267-15-arnd@arndb.de


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>