target/loongarch: Use auto method with PTW feature

target/loongarch: Use auto method with PTW feature

PTW is short for page table walker, it is hardware page table walker
function. With PTW supported, hardware MMU will parse page table
table and update TLB entries automatically.

This patch adds type OnOffAuto for PTW feature setting.

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