LoongArch: Adjust SETUP_SLEEP and SETUP_WAKEUP

SETUP_SLEEP should only save the GPR context, which is symmetric to
SETUP_WAKEUP, so move the acpi_saved_sp handling out of SETUP_SLEEP.

Move "addi.d  sp, sp, PT_SIZE" into SETUP_WAKEUP for the same reason.

No functional changes.

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