refactor(cpufeat): rename ENABLE_SPE_FOR_LOWER_ELS to ENABLE_SPE_FOR_NS

At the moment we hardcode the SPE functionality to be available on the
non-secure side only, by setting MDCR_EL2.E2PB accordingly.

This should be reflected in the feature selection symbol, so rename that
to ENABLE_SPE_FOR_NS, to make it clearer that SPE is not supported in
the secure world.

Change-Id: I3f9b48eab1a45d6ccfcbb9c90a11eeb66867ad9a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/arm/board/tc/platform.mk b/plat/arm/board/tc/platform.mk
index 05dca91..5f4148c 100644
--- a/plat/arm/board/tc/platform.mk
+++ b/plat/arm/board/tc/platform.mk
@@ -161,7 +161,7 @@
 
 override CTX_INCLUDE_PAUTH_REGS	:= 1
 
-override ENABLE_SPE_FOR_LOWER_ELS	:= 0
+override ENABLE_SPE_FOR_NS	:= 0
 
 override ENABLE_AMU := 1
 override ENABLE_AMU_AUXILIARY_COUNTERS := 1