MIPS: u-boot.lds: add relocation specific sections

This section contain the table needed for dynamic
relocation. Also provide symbols for the relocation
code to access the table.

Discard all sections which are not needed in the final
ELF binary and U-Boot image. Section .dynsym cannot be
discarded or GNU ld crashes otherwise. This section
will be stripped by GNU objcpy in a later patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>