RiscVPlatformPkg/RiscVSpecialPlatformLib: Rename module name

Rename RiscVSpecialPlatformLib to RiscVSpecialPlatformLibNull
because this is the NULL instance for  RiscVPlatformPkg.

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>