Platform/RISC-V: Add NULL library instance of RiscVSpecialPlatformLib

This is the library instance to provide platform_override for the
special RISC-V platform. This module incorporates with
OpensbiPlatformLib and RISC-V Opensbi library.

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