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

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>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>