gpio: Remove dynamic allocation from populate_parent_alloc_arg()

The gpiolib is unique in the way it uses intermediate fwspecs
when feeding an interrupt specifier to the parent domain, as it
relies on the populate_parent_alloc_arg() callback to perform
a dynamic allocation.

This is pretty inefficient (we free the structure almost immediately),
and the only reason this isn't a stack allocation is that our
ThunderX friend uses MSIs rather than a FW-constructed structure.

Let's solve it by providing a new type composed of the union
of a struct irq_fwspec and a msi_info_t, which satisfies both
requirements. This allows us to use a stack allocation, and we
can move the handful of users to this new scheme.

Also perform some additional cleanup, such as getting rid of the
stub versions of the irq_domain_translate_*cell helpers, which
are never used when CONFIG_IRQ_DOMAIN_HIERARCHY isn't selected.

Tested on a Tegra186.

Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Cc: Daniel Palmer <[email protected]>
Cc: Romain Perier <[email protected]>
Cc: Bartosz Golaszewski <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Jonathan Hunter <[email protected]>
Cc: Robert Richter <[email protected]>
Cc: Nobuhiro Iwamatsu <[email protected]>
Cc: Andy Gross <[email protected]>
Cc: Bjorn Andersson <[email protected]>
Acked-by: Bartosz Golaszewski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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