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

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
diff --git a/board/gth2/u-boot.lds b/board/gth2/u-boot.lds
index 0ab42db..90432cb 100644
--- a/board/gth2/u-boot.lds
+++ b/board/gth2/u-boot.lds
@@ -43,7 +43,8 @@
 	. = ALIGN(4);
 	.data  : { *(.data) }
 
-	_gp = ALIGN(16);
+	. = .;
+	_gp = ALIGN(16) + 0x7ff0;
 
 	.got : {
 	  __got_start = .;