irqchip/imx-irqsteer: Add runtime PM support

There are now SoCs that integrate the irqsteer controller within
a separate power domain. In order to allow this domain to be
powered down when not needed, add runtime PM support to the driver.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220406163701.1277930-2-l.stach@pengutronix.de