ghes-bert: replace pfn_valid() with memblock_is_map_memory()

After commit 3f7b1a87

 (arm64/mm: drop HAVE_ARCH_PFN_VALID), function
pfn_valid() in arch/arm64/mm/init.c was removed, we should not use pfn_valid()
to ensure presence of linear map. Replace it with memblock_is_map_memory()
to avoid memory mapping error.

Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>