MIPS: BCM63XX: Hook up plat_ioremap to intercept soc registers remapping.

Internal SOC registers can be directly accessed, no need to waste a
TLB entry.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2890/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>