MIPS: make size of relocation table fixed but configurable

MIPS: make size of relocation table fixed but configurable

Currently the size of the relocation table will be shrunk
to the actual size needed. Although this gives a maximal
space saving, it messes up the _end symbol. This breaks
features like appended DTBs because the _end symbol doesn't
point to the real end of the U-Boot binary.

Remove the size shrinking and make the size of the relocation
table fixed but configurable. This follows the Linux approach
and the user can adjust the size to his needs.

Also rename the relocation table section from .rel to .data.reloc
to follow the Linux approach and to avoid ambiguities with the
.rel.* sections added by the linker.

Reported-by: Lars Povlsen <lars.povlsen@microsemi.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>