regmap: irq: add ack_invert flag for chips using cleared bits as ack

An user will be CSR SiRFSoC ARM chips.

Signed-off-by: Guo Zeng <Guo.Zeng@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>