MIPS: add unified u-boot.lds file

The patch adds an unified linker script file which
can be used for all currently supported MIPS targets.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Xiangfu Liu <xiangfu@openmobilefree.net>
Acked-by: Stefan Roese <sr@denx.de>