ARM: sunxi: Select PM_OPP

Device frequency scaling is implemented through devfreq in the kernel,
which requires CONFIG_PM_OPP.

Let's select it.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/mach-sunxi/Kconfig

@@ -6,6 +6,7 @@ menuconfig ARCH_SUNXI
 	select GENERIC_IRQ_CHIP
 	select GPIOLIB
 	select PINCTRL
+	select PM_OPP
 	select SUN4I_TIMER
 	select RESET_CONTROLLER