regmap-irq: Add driver callback to configure virtual regs

Enable drivers to configure and modify "virtual" registers, which are
non-standard registers that further configure irq type on some devices.
Since they are non-standard, enable drivers to configure them according
to their particular idiosyncrasies by specifying an optional callback
function while registering with the framework.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Link: https://lore.kernel.org/r/07e058cdec2297d15c95c825aa0263064d962d5a.1616613838.git.gurus@codeaurora.org


Signed-off-by: Mark Brown <broonie@kernel.org>