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>