RiscVPlatformPkg/Sec: Separate EDK2 Opensbi platform hook.

Separate EDK2 Opensbi platform operations hooks from
Secmain as an individual library which can be override
by OEM platform.

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>