SoC platform removal

There are a lot of platforms that have not seen any interesting code
changes in the past five years or more.

I made a list and asked around which ones are no longer in use [1], and
received confirmation about six ARM platforms and the TI C6x architecture
that have all reached the end of their life upstream, with no known
users remaining:

 - efm32 -- added in 2011, first Cortex-M, no notable changes after 2013
 - picoxcell -- added in 2011, abandoned after 2012 acquisition
 - prima2 -- added in 20111, no notable changes since 2015
 - tango -- added in 2015, sporadic changes until 2017, but abandoned
 - u300 -- added in 2009, no notable changes since 2013
 - zx --added in 2015 for both 32, 2017 for 64 bit, no notable changes
 - arch/c6x -- added in 2011, but work stalled soon after that

A number of other platforms on the original list turned out to still
have users. In some cases there are out-of-tree patches and users
that plan to contribute them in the future, in other cases the code
is complete and works reliably.

[1] https://lore.kernel.org/lkml/CAK8P3a2DZ8xQp7R=H=wewHnT2=a_=M53QsZOueMVEf7tOZLKNg@mail.gmail.com/

Signed-off-by: Arnd Bergmann <arnd@arndb.de>