README: Document 32-bit / 64-bit images build

At present the exact details of building 32-bit or 64-bit OpenSBI
images are missing in the docs. This adds the missing part.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Acked-by: Anup Patel <anup.patel@wdc.com>