README.md: Improve project copyright information

Over-time a lot of organizations and individuals have contributed to
the OpenSBI project so let us add copyright RISC-V International to
respect the contributions from all RISC-V members.

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