MIPS: CPC: Provide default mips_cpc_default_phys_base to ignore CPC

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>

arch/mips/kernel/mips-cpc.c

@@ -21,6 +21,11 @@ static DEFINE_PER_CPU_ALIGNED(spinlock_t, cpc_core_lock);
 
 static DEFINE_PER_CPU_ALIGNED(unsigned long, cpc_core_lock_flags);
 
+phys_addr_t __weak mips_cpc_default_phys_base(void)
+{
+	return 0;
+}
+
 /**
  * mips_cpc_phys_base - retrieve the physical base address of the CPC
  *
@@ -43,8 +48,12 @@ static phys_addr_t mips_cpc_phys_base(void)
 	if (cpc_base & CM_GCR_CPC_BASE_CPCEN_MSK)
 		return cpc_base & CM_GCR_CPC_BASE_CPCBASE_MSK;
 
-	/* Otherwise, give it the default address & enable it */
+	/* Otherwise, use the default address */
 	cpc_base = mips_cpc_default_phys_base();
+	if (!cpc_base)
+		return cpc_base;
+
+	/* Enable the CPC, mapped at the default address */
 	write_gcr_cpc_base(cpc_base | CM_GCR_CPC_BASE_CPCEN_MSK);
 	return cpc_base;
 }