microblaze: Fix missing NR_CPUS in menuconfig

The time Kconfig expects that NR_CPUS is defined.

This patch remove this config warning:
"kernel/time/Kconfig:163:warning: range is invalid"
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/Kconfig

@@ -129,6 +129,10 @@ endmenu
 
 menu "Kernel features"
 
+config NR_CPUS
+	int
+	default "1"
+
 config ADVANCED_OPTIONS
 	bool "Prompt for advanced kernel configuration options"
 	help