iommu/apple-dart: Use readl instead of readl_relaxed for consistency

While the readl_relaxed in apple_dart_suspend is correct the rest of the
driver uses the non-relaxed variants everywhere and the single
readl_relaxed is inconsistent and possibly confusing.

Signed-off-by: Sven Peter <sven@svenpeter.dev>
Acked-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Link: https://lore.kernel.org/r/20231126162009.17934-1-sven@svenpeter.dev


Signed-off-by: Joerg Roedel <jroedel@suse.de>