LoongArch: Add debugfs entries to switch SFB/TSO state

We need to switch SFB (Store Fill Buffer) and TSO (Total Store Order)
state at runtime to debug memory management and KVM virtualization, so
add two debugfs entries "sfb_state" and "tso_state" under the directory
/sys/kernel/debug/loongarch.

Query SFB:
cat /sys/kernel/debug/loongarch/sfb_state

Enable SFB:
echo 1 > /sys/kernel/debug/loongarch/sfb_state

Disable SFB:
echo 0 > /sys/kernel/debug/loongarch/sfb_state

Query TSO:
cat /sys/kernel/debug/loongarch/tso_state

Switch TSO:
echo [TSO] > /sys/kernel/debug/loongarch/tso_state

Available [TSO] states:
0 (No Load No Store)    1 (All Load No Store)   3 (Same Load No Store)
4 (No Load All Store)   5 (All Load All Store)  7 (Same Load All Store)

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>