Add macro to check whether the CPU implements an EL

Replace all instances of checks with the new macro.

Change-Id: I0eec39b9376475a1a9707a3115de9d36f88f8a2a
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/lib/el3_runtime/aarch64/context_mgmt.c b/lib/el3_runtime/aarch64/context_mgmt.c
index 5cce879..b16e55d 100644
--- a/lib/el3_runtime/aarch64/context_mgmt.c
+++ b/lib/el3_runtime/aarch64/context_mgmt.c
@@ -229,8 +229,7 @@
 			sctlr_elx &= ~SCTLR_EE_BIT;
 			sctlr_elx |= SCTLR_EL2_RES1;
 			write_sctlr_el2(sctlr_elx);
-		} else if (read_id_aa64pfr0_el1() &
-			   (ID_AA64PFR0_ELX_MASK << ID_AA64PFR0_EL2_SHIFT)) {
+		} else if (EL_IMPLEMENTED(2)) {
 			/* EL2 present but unused, need to disable safely */
 
 			/* HCR_EL2 = 0, except RW bit set to match SCR_EL3 */