Add platform API for reset handling

This patch adds an optional platform API (plat_reset_handler) which allows the
platform to perform any actions immediately after a cold or warm reset
e.g. implement errata workarounds. The function is called with MMU and caches
turned off. This API is weakly defined and does nothing by default but can be
overriden by a platform with a strong definition.

Change-Id: Ib0acdccbd24bc756528a8bd647df21e8d59707ff
diff --git a/plat/common/aarch64/platform_helpers.S b/plat/common/aarch64/platform_helpers.S
index 6dc4ec6..1229ead 100644
--- a/plat/common/aarch64/platform_helpers.S
+++ b/plat/common/aarch64/platform_helpers.S
@@ -38,6 +38,7 @@
 	.weak	plat_report_exception
 	.weak	plat_crash_console_init
 	.weak	plat_crash_console_putc
+	.weak	plat_reset_handler
 
 	/* -----------------------------------------------------
 	 *  int platform_get_core_pos(int mpidr);
@@ -84,3 +85,11 @@
 	 */
 func plat_crash_console_putc
 	ret
+
+	/* -----------------------------------------------------
+	 * Placeholder function which should be redefined by
+	 * each platform. This function should preserve x10.
+	 * -----------------------------------------------------
+	 */
+func plat_reset_handler
+	ret