ARM: remove sirf prima2/atlas platforms

The SiRF Prima2 and Atlas platform code was contributed by Cambridge
Silicon Radio (CSR) after aquiring the original SiRF company, and
maintained by Barry Song. CSR was subsequently acquired by Qualcomm,
who no longer have an interest in maintaining the SoC platform but
instead have released more recent SoCs for the same market in the
Snapdragon family.

As Barry is no longer working for the company, nobody else there
wants to maintain it, and there are no third-party users, the
best way forward seems to be to completely remove it.

Thanks to Barry for maintaining the platform for the past ten years.

Cc: Barry Song <baohua@kernel.org>
Link: https://lore.kernel.org/lkml/c969392572604b98bcb3be44048c3165@hisilicon.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>