nios2: Add missing NR_CPUS to Kconfig

kernel/time/Kconfig expects that NR_CPUS is defined.

This fixes the following config warning:
"kernel/time/Kconfig:163:warning: range is invalid"

Follow 4cbbbb43 ("microblaze: Fix missing NR_CPUS in menuconfig")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>

arch/nios2/Kconfig

@@ -79,6 +79,10 @@ menu "Processor type and features"
 config MMU
 	def_bool y
 
+config NR_CPUS
+	int
+	default "1"
+
 config NIOS2_ALIGNMENT_TRAP
 	bool "Catch alignment trap"
 	default y