sw64: mm: use memblock to find the end of memory

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56TYY



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Use memblock_end_of_DRAM() instead of PFN_PHYS(max_low_pfn) in
process_memmap to find a more accurate end of memory.

Signed-off-by: Mao Minkai <maominkai@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>