[MIPS] u-boot.lds: Define _gp in a standard manner

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
diff --git a/examples/mips.lds b/examples/mips.lds
index 3bd573c..a770728 100644
--- a/examples/mips.lds
+++ b/examples/mips.lds
@@ -39,7 +39,8 @@
 	. = ALIGN(4);
 	.data  : { *(.data) }
 
-	_gp = ALIGN(16);
+	. = .;
+	_gp = ALIGN(16) + 0x7ff0;
 
 	.got : {
 	  __got_start = .;