ProcessorPkg/opensbi: Update opensbi library

Update opensbi library to a731c7e36988c3308e1978ecde491f2f6182d490,
which is based on v0.9.

Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>