MdePkg/Include: RISC-V definitions.

MdePkg/Include: RISC-V definitions.

Add RISC-V processor related definitions.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2672

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Co-authored-by: Gilbert Chen <gilbert.chen@hpe.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Gilbert Chen <gilbert.chen@hpe.com>