armv7: Fix entry point in ram stage.

Eliminate the warning message:
 ld: warning: cannot find entry symbol _start; defaulting to 040000000

The "_start" from c_start.S is deprecated so we need to define entry
point again in link description file.

Change-Id: I174428faa2e7f08cd91fe96a53e6efea9dc3634e
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: http://review.coreboot.org/2258
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>

src/arch/armv7/coreboot_ram.ld

@@ -32,6 +32,7 @@ SECTIONS
 	 */
 	.text : {
 		_text = .;
+		_start = .;
 		*(.text.stage_entry.armv7);
 		*(.text);
 		*(.text.*);