mips: mtmips: add SPL support

mips: mtmips: add SPL support

This patch adds SPL support for mtmips platform. The lowlevel architecture
is split into SPL and the rest parts are built into a memory loadable
u-boot image. Optional SPL_DM and OF_CONTROL are also supported.

The increment of size is very small (< 10 KiB) if SPL_DM and OF_CONTROL are
not enabled and the memory bootable u-boot (u-boot.img) is generated
automatically so there is not need to add a separate config for it.

A lzma compressed payload (u-boot-lzma.img) is also generated and it will
be combined with u-boot-spl.bin to form the unified ROM bootable binary
u-boot-mtmips.bin.

A spl loader is added to support uncompress the payload.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>