arm: ioremap: don't abuse pfn_valid() to check if pfn is in RAM

The semantics of pfn_valid() is to check presence of the memory map for a
PFN and not whether a PFN is in RAM. The memory map may be present for a
hole in the physical memory and if such hole corresponds to an MMIO range,
__arm_ioremap_pfn_caller() will produce a WARN() and fail:

Use memblock_is_map_memory() instead of pfn_valid() to check if a PFN is in
RAM or not.
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