ARM: Enable RTIC MPGen

Added RTIC environmental variable check. If set, this will trigger
RTIC MPGen during the kernel build. MPGen generates and embeds the
kernel MP.s (measurement parameters) to the vmlinux. It has to be
called during the kernel build, before vmlinux is generated.
RTIC MP.s to be consumed by the RTIC TA.

Change-Id: Id070d0c1960c38d286c31c55d45a6c448348d60d
Signed-off-by: Preeti Nagar <pnagar@codeaurora.org>