ARM: remove tango platform

The smp8758 (tango4) SoC was the last generation of set-top-box chips
to come out of Sigma Designs, and support was added by Marc Gonzalez
and Måns Rullgård between 2015 and 2017, before the company went out of
business and the products were abandoned.

The chip is used in some set-top-boxes such as the Popcorn Hour A-500,
which could have seen some adoption by hobbyists. This has not happened
in the past four years, and support for the more widely used MIPS based
SoCs was never merged at all.

Thanks to Marc and Måns for maintaining for the past years even after the
death of the platform.

Cc: Marc Gonzalez <marc.w.gonzalez@free.fr>
Cc: Mans Rullgard <mans@mansr.com>
Link: https://lore.kernel.org/lkml/2d643ebc-09af-a809-eb3f-2aec8ecee501@free.fr/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>