irqchip/gic: Switch to dynamic chip name output

The last dynamic aspect of the GIC's irq_chip structure is the
name that is associated to it.

Move the output of that name to the relevant callback, which
allows us to do a bit of cleanup and mark the structures const.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220209162607.1118325-4-maz@kernel.org