MIPS: provide a default u-boot-spl.lds

Provide a default linker script for SPL binaries. Start address
and size of text section and BSS section are configurable. All
sections are arranged in a way that only relevant sections are
kept in the code section for maximum size reduction. All other
sections are kept but moved outside the code section to help
with debugging.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>