[MIPS] Kconfig: Fix configuration warning by hardwiring HOTPLUG_CPU to n.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -697,6 +697,10 @@ config GENERIC_ISA_DMA
 	bool
 	select ZONE_DMA
 
+config HOTPLUG_CPU
+	bool
+	default n
+
 config I8259
 	bool