gpiolib: Add support for the irqdomain which doesn't use irq_fwspec as arg

gpiolib: Add support for the irqdomain which doesn't use irq_fwspec as arg

Some gpio's parent irqdomain may not use the struct irq_fwspec as
argument, such as msi irqdomain. So rename the callback
populate_parent_fwspec() to populate_parent_alloc_arg() and make it
allocate and populate the specific struct which is needed by the
parent irqdomain.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Link: https://lore.kernel.org/r/20200114082821.14015-3-haokexin@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>