MIPS: Malta: Implement mips_cdmm_phys_base()

MIPS: Malta: Implement mips_cdmm_phys_base()

Implement mips_cdmm_phys_base() for Malta, returning the physical base
address 0x1fc10000 which is "typically unused".

This allows the Common Device Memory Map (CDMM) region to be mapped, and
devices in that region (such as the Fast Debug Channel (FDC) hardware
for communication over EJTAG) to be discovered.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9177/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>