MdePkg/Include: Add RISC-V related definitions EDK2 CI.

MdePkg/Include: Add RISC-V related definitions EDK2 CI.

HTTP/PXE boot RISC-V related definitions for EDK2 CI.

BZ:2562:
https://bugzilla.tianocore.org/show_bug.cgi?id=2562

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Gilbert Chen <gilbert.chen@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>