MIPS: Loongson64: Clean up numa.c

(1) Replace nid_to_addroffset() with nid_to_addrbase() and then remove the
related useless code.

(2) Since end_pfn = start_pfn + node_psize, use "node_psize" instead of
"end_pfn - start_pfn" to avoid the redundant calculation.

(3) After commit 6fbde6b4

 ("MIPS: Loongson64: Move files to the
top-level directory"), CONFIG_ZONE_DMA32 is always set for Loongson64
due to MACH_LOONGSON64 selects ZONE_DMA32, so no need to use ifdef any
more, just remove it.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>