MIPS: use 3-level pgtable for 64KB page size on MIPS_VA_BITS_48

MIPS: use 3-level pgtable for 64KB page size on MIPS_VA_BITS_48

It hangup when booting Loongson 3A1000 with BOTH
CONFIG_PAGE_SIZE_64KB and CONFIG_MIPS_VA_BITS_48, that it turn
out to use 2-level pgtable instead of 3-level. 64KB page size
with 2-level pgtable only cover 42 bits VA, use 3-level pgtable
to cover all 48 bits VA(55 bits)

Fixes: 1607126f99bb ("MIPS64: Support of at least 48 bits of SEGBITS)
Signed-off-by: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>