cppc_acpi: print error message if CPPC is unsupported

The amd-pstate driver can fail when _CPC objects are not supported by
the CPU. However, the current error message is ambiguous (see below) and
there is no clear way for attributing the failure of the amd-pstate
driver to the lack of CPPC support.

[    0.477523] amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled

Fix this by adding an debug message to notify the user if the amd-pstate
driver failed to load due to CPPC not be supported by the CPU

Reviewed-by: Mario Limonciello <[email protected]>
Reviewed-by: Gautham R. Shenoy <[email protected]>
Tested-by: Dhananjay Ugwekar <[email protected]>
Signed-off-by: Perry Yuan <[email protected]>
Acked-by: Huang Rui <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Perry Yuan

drivers/acpi/cppc_acpi.c

@@ -686,8 +686,10 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
 
 	if (!osc_sb_cppc2_support_acked) {
 		pr_debug("CPPC v2 _OSC not acked\n");
-		if (!cpc_supported_by_cpu())
+		if (!cpc_supported_by_cpu()) {
+			pr_debug("CPPC is not supported by the CPU\n");
 			return -ENODEV;
+		}
 	}
 
 	/* Parse the ACPI _CPC table for this CPU. */