iommu/arm-smmu-v3: Don't display an error when IRQ lines are missing

Since commit 7723f4c5 ("driver core: platform: Add an error message
to platform_get_irq*()"), platform_get_irq_byname() displays an error
when the IRQ isn't found. Since the SMMUv3 driver uses that function to
query which interrupt method is available, the message is now displayed
during boot for any SMMUv3 that doesn't implement the combined
interrupt, or that implements MSIs.

[   20.700337] arm-smmu-v3 arm-smmu-v3.7.auto: IRQ combined not found
[   20.706508] arm-smmu-v3 arm-smmu-v3.7.auto: IRQ eventq not found
[   20.712503] arm-smmu-v3 arm-smmu-v3.7.auto: IRQ priq not found
[   20.718325] arm-smmu-v3 arm-smmu-v3.7.auto: IRQ gerror not found

Use platform_get_irq_byname_optional() to avoid displaying a spurious
error.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>