Unmask SError interrupt and clear SCR_EL3.EA bit

This patch disables routing of external aborts from lower exception levels to
EL3 and ensures that a SError interrupt generated as a result of execution in
EL3 is taken locally instead of a lower exception level.

The SError interrupt is enabled in the TSP code only when the operation has not
been directly initiated by the normal world. This is to prevent the possibility
of an asynchronous external abort which originated in normal world from being
taken when execution is in S-EL1.

Fixes ARM-software/tf-issues#153

Change-Id: I157b996c75996d12fd86d27e98bc73dd8bce6cd5
diff --git a/bl1/aarch64/bl1_exceptions.S b/bl1/aarch64/bl1_exceptions.S
index 8ab9df8..13b34b7 100644
--- a/bl1/aarch64/bl1_exceptions.S
+++ b/bl1/aarch64/bl1_exceptions.S
@@ -112,6 +112,9 @@
 	 */
 	.align	7
 SynchronousExceptionA64:
+	/* Enable the SError interrupt */
+	msr	daifclr, #DAIF_ABT_BIT
+
 	/* ------------------------------------------------
 	 * Only a single SMC exception from BL2 to ask
 	 * BL1 to pass EL3 control to BL31 is expected