Provide a default implementation of mips_cpc_default_phys_base() which
simply returns 0, and adjust mips_cpc_phys_base() to allow for
mips_cpc_default_phys_base() returning 0. This allows kernels which
include CPC support to be built without platform code & simply ignore
the CPC if it wasn't already enabled by the bootloader.

This fixes link failures such as the following from generic defconfigs:

   arch/mips/built-in.o: In function `mips_cpc_phys_base':
   arch/mips/kernel/mips-cpc.c:47: undefined reference to `mips_cpc_default_phys_base'

[ralf@linux-mips.org: changed prototype for coding style compliance.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14401/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>