PlatformPkg/SecMain: RISC-V SecMain module.

SecMain module for RISC-V platform.
This was cloned from OpenSBI fw_base.S
(RiscVPkg/Library/RiscVOpensbiLib/opensbi/firmware/) and revised to
edk2 framework.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Co-authored-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Co-authored-by: Gilbert Chen <gilbert.chen@hpe.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Gilbert Chen <gilbert.chen@hpe.com>