docs: Add OpenSBI logo and use it in the top-level README.md

We do have an official OpenSBI logo which was designed few months ago
and was also approved by RISC-V International. Lets add this logo
under docs and also use it in the top-level README.md

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>