LoongArch: Fix Kconfig item and left code related to CRASH_CORE

In commit 85fcde40 ("kexec: split crashkernel reservation code
out from crash_core.c"), crashkernel reservation code is split out from
crash_core.c, and add CRASH_RESERVE to control it. And also rename each
ARCH's <asm/crash_core.h> to <asm/crash_reserve.h> accordingly.

But the relevant part in LoongArch is missed. Do it now.

Fixes: 85fcde40

 ("kexec: split crashkernel reservation code out from crash_core.c")
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>