clocksource/drivers/atlas: Remove sirf atlas driver

The CSR SiRF prima2/atlas platforms are getting removed, so this driver
is no longer needed.

Cc: Barry Song <baohua@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Barry Song <baohua@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210120131559.1971359-4-arnd@kernel.org