AArch64: Introduce External Abort handling

At present, any External Abort routed to EL3 is reported as an unhandled
exception and cause a panic. This patch enables ARM Trusted Firmware to
handle External Aborts routed to EL3.

With this patch, when an External Abort is received at EL3, its handling
is delegated to plat_ea_handler() function. Platforms can provide their
own implementation of this function. This patch adds a weak definition
of the said function that prints out a message and just panics.

In order to support handling External Aborts at EL3, the build option
HANDLE_EA_EL3_FIRST must be set to 1.

Before this patch, HANDLE_EA_EL3_FIRST wasn't passed down to
compilation; this patch fixes that too.

Change-Id: I4d07b7e65eb191ff72d63b909ae9512478cd01a1
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index aa181c8..cd17a00 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -124,6 +124,9 @@
 void plat_sdei_handle_masked_trigger(uint64_t mpidr, unsigned int intr);
 #endif
 
+void plat_ea_handler(unsigned int ea_reason, uint64_t syndrome, void *cookie,
+		void *handle, uint64_t flags);
+
 /*
  * The following function is mandatory when the
  * firmware update feature is used.