iommu/mediatek: Check 4GB mode by reading infracfg

In previous discussion [1] and [2], we found that it is risky to
use max_pfn or totalram_pages to tell if 4GB mode is enabled.

Check 4GB mode by reading infracfg register, remove the usage
of the un-exported symbol max_pfn.

This is a step towards building mtk_iommu as a kernel module.

[1] https://lore.kernel.org/lkml/[email protected]/
[2] https://lore.kernel.org/lkml/[email protected]/
[3] https://lore.kernel.org/lkml/20200715205120.GA778876@bogus/

Cc: Mike Rapoport <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Yong Wu <[email protected]>
Cc: Yingjoe Chen <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: Joerg Roedel <[email protected]>
Reviewed-by: Matthias Brugger <[email protected]>
Signed-off-by: Miles Chen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

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