LoongArch: Cleanup reset routines with new API

Cleanup reset routines by using new do_kernel_power_off() instead of old
pm_power_off(), and then simplify the whole file (reset.c) organization
by inlining some functions. This cleanup also fix a poweroff error if EFI
runtime is disabled.

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