firmware: Remove redundant add instruction from trap restore path

The "add sp, a0, zero" instruction in the trap restore path is redundant
and can be avoided if TRAP_RESTORE_xyz() assembly macros use a0 as the
base register instead of sp.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>