include: sbi: Don't pack struct sbi_platform and sbi_platform_operations

We don't need to pack struct sbi_platform and sbi_platform_operations
because GCC ensures member offsets match member data type irrespective
to the target system (RV32 or RV64). This also allows GCC to generate
more optimized instruction sequence when accessing members of struct
sbi_platform and struct sbi_platform_operations.

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>