ARM: remove zte zx platform

The ZTE ZX set-top-box SoC platform was added in 2015 by Jun Nie, with
Baoyou Xie and Shawn Guo subsequently becoming maintainers after the
addition of the 64-bit variant.

However, the only machines that were ever supported upstream are the
reference designs, not actual set-top-box devices that would benefit
from this support. All ZTE set-top-boxes from the past few years seem
to be based on third-party SoCs. While there is very little information
about zx296702 and zx296718 on the web, I found some references to other
chips from the same family, such as zx296716 and zx296719, which were
never submitted for upstream support. Finally, there is no support for
the GPU on either of them, with the lima and panfrost device drivers
having been added after work on the zx platform had stopped.

Shawn confirmed that he has not seen any interest in this platform for
the past four years, and that it can be removed.

Thanks to Jun and Shawn for maintaining this platform over the past
five years.

Cc: Jun Nie <jun.nie@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>