ARM: omap2plus_defconfig: use CONFIG_CPUFREQ_DT

CONFIG_GENERIC_CPUFREQ_CPU0 disappeared with commit bbcf0719
("cpufreq: cpu0: rename driver and internals to 'cpufreq_dt'")

Use the renamed CONFIG_CPUFREQ_DT generic driver. It looks like with
v3.18-rc1, commit bbcf0719 and fdc509b1 came in via
different trees causing the resultant v3.18-rc1 to be non-functional for
cpufreq as default supported with omap2plus_defconfig.

Fixes: fdc509b1

 ("ARM: omap2plus_defconfig: Add cpufreq to defconfig")
Cc: <stable@vger.kernel.org> # 3.18
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>