irqchip/mtk-sysirq: Convert to a platform driver

This driver can work as a platform driver. So covert it to a platform
driver.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Hanks Chen <hanks.chen@mediatek.com>
Link: https://lore.kernel.org/r/20200718000637.3632841-4-saravanak@google.com