LoongArch: Cleanup reset routines with new API

mainline inclusion
from mainline-v6.0-rc3
commit da48b67c
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

--------------------------------

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>