OvmfPkg/FdtPciHostBridgeLib: Add RISC-V in the supported arch.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Jiewen Yao <jiewen.yao@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>