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/lib/extensions/pmuv3/aarch64/pmuv3.c b/lib/extensions/pmuv3/aarch64/pmuv3.c
index f83a5ee..fda71aa 100644
--- a/lib/extensions/pmuv3/aarch64/pmuv3.c
+++ b/lib/extensions/pmuv3/aarch64/pmuv3.c
@@ -48,7 +48,7 @@
 	return mdcr_el3;
 }
 
-void pmuv3_disable_el3(void)
+void pmuv3_init_el3(void)
 {
 	u_register_t mdcr_el3 = read_mdcr_el3();