ARM: orion5x: include linux/cpu.h

The linux/cpu.h header is no longer implictly included in this
file, so we need to an #include statement to avoid this build
warning:

arch/arm/mach-orion5x/common.c:339:3: error: implicit declaration of function 'cpu_idle_poll_ctrl' [-Werror=implicit-function-declaration]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>

arch/arm/mach-orion5x/common.c

@@ -19,6 +19,7 @@
 #include <linux/ata_platform.h>
 #include <linux/delay.h>
 #include <linux/clk-provider.h>
+#include <linux/cpu.h>
 #include <net/dsa.h>
 #include <asm/page.h>
 #include <asm/setup.h>