MIPS: Relocatable: Provide plat_post_relocation hook

MIPS: Relocatable: Provide plat_post_relocation hook

This hook provides the platform the chance to perform any required
setup before the boot processor switches to the relocated kernel.
The relocated kernel has been copied and fixed up ready for execution
at this point. Secondary CPUs may wish to switch to it early. There
is also the opportunity for the platform to abort jumping to the
relocated kernel if there is anything wrong with the chosen offset.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Signed-off-by: Steven J. Hill <Steven.Hill@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14651/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>