iommu/rockchip: Don't use platform_get_irq to implicitly count irqs

Till now the Rockchip iommu driver walked through the irq list via
platform_get_irq() until it encountered an ENXIO error. With the
recent change to add a central error message, this always results
in such an error for each iommu on probe and shutdown.

To not confuse people, switch to platform_count_irqs() to get the
actual number of interrupts before walking through them.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>