Maintainers.txt: Add entry for OvmfPkg/RiscVVirt

Maintainers.txt: Add entry for OvmfPkg/RiscVVirt

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4076

RiscVVirt is created to support EDK2 for RISC-V qemu
virt machine platform. Add maintainer entries.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>