irqchip/meson-gpio: Make it possible to build as a module

In order to reduce the kernel Image size on multi-platform distributions,
make it possible to build the Amlogic GPIO IRQ controller as a module
by switching it to a platform driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Saravana Kannan <saravanak@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210902134914.176986-2-narmstrong@baylibre.com