iommu/arm-smmu: Make instance lookup robust

Relying on the driver list was a cute idea for minimising the scope of
our SMMU device lookups, however it turns out to have a subtle flaw. The
SMMU device only gets added to that list after arm_smmu_device_probe()
returns success, so there's actually no way the iommu_device_register()
call from there could ever work as intended, even if it wasn't already
hampered by the fwspec setup not happening early enough.

Switch both arm_smmu_get_by_fwnode() implementations to use a platform
bus lookup instead, which *will* reliably work. Also make sure that we
don't register SMMUv2 instances until we've fully initialised them, to
avoid similar consequences of the lookup now finding a device with no
drvdata. Moving the error returns is also a perfect excuse to streamline
them with dev_err_probe() in the process.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/6d7ce1dc31873abdb75c895fb8bd2097cce098b4.1733406914.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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