efi/loongarch: Use load address to calculate kernel entry address

The efi_relocate_kernel() may load the PIE kernel to anywhere, the
loaded address may not be equal to link address or
EFI_KIMG_PREFERRED_ADDRESS.

Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Wang Yao <wangyao@lemote.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>