mips: add an option to enable u_boot_list section for SPL loaders in u-boot-spl.lds

u_boot_list is not only used by DM, but also by some SPL image load methods
such as spl_nor.c.

This patch adds an option CONFIG_SPL_LOADER_SUPPORT in conjunction with
CONFIG_SPL_DM surrounding the u_boot_list section to make sure SPL image
loaders can be correctly built into u-boot SPL without DM enabled.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/cpu/u-boot-spl.lds b/arch/mips/cpu/u-boot-spl.lds
index e467491..28ea4f2 100644
--- a/arch/mips/cpu/u-boot-spl.lds
+++ b/arch/mips/cpu/u-boot-spl.lds
@@ -27,7 +27,7 @@
 		*(SORT_BY_ALIGNMENT(.sdata*))
 	} > .spl_mem
 
-#ifdef CONFIG_SPL_DM
+#if defined(CONFIG_SPL_DM) || defined(CONFIG_SPL_LOADER_SUPPORT)
 	. = ALIGN(4);
 	.u_boot_list : {
 		KEEP(*(SORT(.u_boot_list*)));