FROMLIST: irqchip: irq-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>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20201020072532.949137-2-narmstrong@baylibre.com
Bug: 179406580
Change-Id: Id8942784089a977cafaf8b5949aeda2b63ac3982
(cherry picked from commit 95d54354)