target/loongarch: Only support 64bit pte width

target/loongarch: Only support 64bit pte width

iFrom LoongArch Reference Manual pte width can be 64bit, 128bit
or more. Instead real hardware only supports 64bit pte width.
For 12bit pte, there is no detail definition for all 128bit
from manual.

Here only 64bit pte width is supported for simplicity, will add
this in later if real hw support it and there is definition for
all the bits from manual.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>