include: sbi: No need to pack struct sbi_trap_regs

All members in struct sbi_trap_regs are of machine word size so
no need to pack this structure. This also allows GCC to generate
more optimized instruction sequence when accessing members of
struct sbi_trap_regs.

Reported-by: Paul Campbell <taniwha@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>