MdePkg: add SBI-based SerialPortLib for RISC-V

These are implementations of SerialPortLib using SBI console services.
- BaseSerialPortLibRiscVSbiLib is appropriate for SEC/PEI (XIP)
  environments
- BaseSerialPortLibRiscVSbiLibRam is appropriate for PrePI/DXE
  environments

Tested with:
- Qemu RiscVVirt (non-DBCN case, backed by UART)
- TinyEMU + RiscVVirt (non-DBCN case, HTIF)
- TinyEMU + RiscVVirt (DBCN case, HTIF)

Cc: Daniel Schaefer <git@danielschaefer.me>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Andrei Warkentin <andrei.warkentin@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

MdePkg/Library/BaseSerialPortLibRiscVSbiLib/Common.h

@@ -0,0 +1,41 @@
+/** @file
+  Serial Port Library backed by SBI console.
+
+  Common functionality shared by PrePiDxeSerialPortLibRiscVSbi and
+  PrePiDxeSerialPortLibRiscVSbiRam implementations.
+
+  Copyright (c) 2023, Intel Corporation. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef SERIAL_PORT_SBI_COMMON_H_
+#define SERIAL_PORT_SBI_COMMON_H_
+
+#include <Base.h>
+#include <Library/SerialPortLib.h>
+#include <Library/BaseRiscVSbiLib.h>
+
+BOOLEAN
+SbiImplementsDbcn (
+  VOID
+  );
+
+BOOLEAN
+SbiImplementsLegacyPutchar (
+  VOID
+  );
+
+UINTN
+SbiLegacyPutchar (
+  IN  UINT8  *Buffer,
+  IN  UINTN  NumberOfBytes
+  );
+
+UINTN
+SbiDbcnWrite (
+  IN  UINT8  *Buffer,
+  IN  UINTN  NumberOfBytes
+  );
+
+#endif /* SERIAL_PORT_SBI_COMMON_H_ */