irqchip: Remove sirfsoc 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: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210120133008.2421897-3-arnd@kernel.org