regmap: irq: add support for chips who have separate unmask registers

Some chips have separate unmask registers from mask registers for
some consideration of concurrency SMP write performance. And this
patch adds a flag for it.

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>