chore(rme): add make rule for SPD=spmd

ENABLE_RME is set then SPD must either be spmd or it should not be set.
Add a rule to assert this.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I0556e7b0e55b04c3a8e4c20c991fbbc30486570c
diff --git a/Makefile b/Makefile
index 5306ddf..d79fa96 100644
--- a/Makefile
+++ b/Makefile
@@ -642,6 +642,11 @@
 ifeq ($(SPMC_AT_EL3),1)
 	$(error SPMC_AT_EL3 and ENABLE_RME cannot both be enabled.)
 endif
+ifneq (${SPD}, none)
+ifneq (${SPD}, spmd)
+       $(error ENABLE_RME is incompatible with SPD=${SPD}. Use SPD=spmd)
+endif
+endif
 include services/std_svc/rmmd/rmmd.mk
 $(warning "RME is an experimental feature")
 endif