OvmfPkg/QemuFwCfgLibMmio: Add RISC-V arch support

OvmfPkg/QemuFwCfgLibMmio: Add RISC-V arch support

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>