MIPS: Reserve space for relocation table

When CONFIG_RELOCATABLE is enabled, add a new section in the memory map
to be filled with relocation data.

CONFIG_RELOCATION_TABLE_SIZE allows the amount of space reserved to be
adjusted if necessary.

The relocs tool will populate this reserved space with relocation
information. The space is reserved within the elf by filling it with
0's, and an invalid entry is left at the start of the space such that
kernel relocation will be aborted if the table is empty.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Aaro Koskinen <aaro.koskinen@nokia.com>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12982/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>