ARM: ux500: u8500_defconfig: add missing cpuidle option

With the commit d3f2950f, the option
ARM_U8500_CPUIDLE was added to the Kconfig but not reflected in the
default config file, hence the cpuidle driver is no longer enabled
since this commit.

Enable it again by adding the missing option in the default config file.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>