ARM: omap2plus_defconfig: Add cpufreq to defconfig

Note that we can now use the CONFIG_GENERIC_CPUFREQ_CPU0,
so let's only enable that. Let's use CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND
as suggested by Nishant.

And also let's enable thermal as explained by Nishant Menon:

Many TI SoCs using Highest frequency is not really too nice of an idea for
long periods of time. And not everything is upstream to support things
optimially - example avs class 0, 1.5 ABB consolidation with cpufreq etc..
We definitely need thermal enabled as well for device safety needs.

[tony@atomide.com: updated per Nishant's suggestions]
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>