ARM: OMAP2+: Enable CPUidle in omap2plus_defconfig

Enable CPUidle so it's easier for maintainers to notice
if some future code changes cause regressions.

Cc: Kevin Hilman <khilman@linaro.org>
Cc: Nishanth Menon <nm@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>