firmware: Minor optimization for relocate

The t3 register stores the address of _load_end. If relocation is not
required, it is unnecessary to calculate the address of _load_end.

This can reduce the operation time of two instructions.

Signed-off-by: Dongdong Zhang <zhangdongdong@eswincomputing.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>