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/pmuv3.h b/include/lib/extensions/pmuv3.h
index 5d5d055..62fee7b 100644
--- a/include/lib/extensions/pmuv3.h
+++ b/include/lib/extensions/pmuv3.h
@@ -9,7 +9,7 @@
 
 #include <context.h>
 
-void pmuv3_disable_el3(void);
+void pmuv3_init_el3(void);
 
 #ifdef __aarch64__
 void pmuv3_enable(cpu_context_t *ctx);