nb/amd: add IS_ENABLED() around Kconfig symbol references

Some of these can be changed from #if to if(), but that will happen
in a follow-on commmit.

Change-Id: I763cbbc31dcd4cdd128c04793a742ab6daaf5f0c
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20345
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/northbridge/amd/amdfam10/debug.h

@@ -32,7 +32,7 @@ void dump_pci_device_index(u32 dev, u32 index_reg, u32 type, u32 length);
 void dump_pci_devices(void);
 void dump_pci_devices_on_bus(u32 busn);
 
-#if CONFIG_DEBUG_SMBUS
+#if IS_ENABLED(CONFIG_DEBUG_SMBUS)
 void dump_spd_registers(const struct mem_controller *ctrl);
 void dump_smbus_registers(void);
 #endif