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/bl31/bl31_main.c b/bl31/bl31_main.c
index c74b72b..55d0bd9 100644
--- a/bl31/bl31_main.c
+++ b/bl31/bl31_main.c
@@ -172,8 +172,7 @@
 	 * Ensure that the build flag to save AArch32 system registers in CPU
 	 * context is not set for AArch64-only platforms.
 	 */
-	if (((read_id_aa64pfr0_el1() >> ID_AA64PFR0_EL1_SHIFT)
-			& ID_AA64PFR0_ELX_MASK) == 0x1) {
+	if (EL_IMPLEMENTED(1) == EL_IMPL_A64ONLY) {
 		ERROR("EL1 supports AArch64-only. Please set build flag "
 				"CTX_INCLUDE_AARCH32_REGS = 0");
 		panic();