ARM: remove w90x900 platform

This removes the old Winbond w90x900 platform, also known
as Nuvoton NUC900. Wan Zongshun originally contributed
the port and maintained it since then.

From all I can tell, this platform is no longer being used
with modern kernels, based on various indications:

- The supported chips (nuc910/950/960) are no longer marketed
  by the manufacturer

- Newer chips from the same family (nuc97x, nuc980, n329x)
  that are still marketed have Linux BSPs but those were never
  submitted for upstream inclusion.

- The last patch from the platform maintainer was in 2011.

- All patches to w90x900 platform specific files afterwards
  are cleanups that were apparently done without access to
  test hardware.

- Both the website and the email address listed in the
  MAINTAINERS have become unreachable.

Link: https://lore.kernel.org/r/20190809202749.742267-17-arnd@arndb.de


Cc: "Wanzongshun (Vincent)" <wanzongshun@huawei.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>