refactor(cpufeat): separate the EL2 and EL3 enablement code

Combining the EL2 and EL3 enablement code necessitates that it must be
called at el3_exit, which is the only place with enough context to make
the decision of what needs to be set.
Decouple them to allow them to be called from elsewhere.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I147764c42771e7d4100699ec8fae98dac0a505c0
diff --git a/include/lib/extensions/brbe.h b/include/lib/extensions/brbe.h
index 9ee2444..194efba 100644
--- a/include/lib/extensions/brbe.h
+++ b/include/lib/extensions/brbe.h
@@ -8,9 +8,9 @@
 #define BRBE_H
 
 #if ENABLE_BRBE_FOR_NS
-void brbe_enable(void);
+void brbe_init_el3(void);
 #else
-static inline void brbe_enable(void)
+static inline void brbe_init_el3(void)
 {
 }
 #endif /* ENABLE_BRBE_FOR_NS */