MdePkg: add definition of LINUX_EFI_INITRD_MEDIA_GUID

MdePkg: add definition of LINUX_EFI_INITRD_MEDIA_GUID

Add LINUX_EFI_INITRD_MEDIA_GUID to our collection of GUID definitions,
it can be used in a media device path to specify a Linux style initrd
that can be loaded by the OS using the LoadFile2 protocol.

Move these defines to MdePkg from OvmfPkg as these are relevant to
non-OVMF targets as well.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2564
Signed-off-by: Jeff Brasen <jbrasen@nvidia.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>