ARM: omap2plus_defconfig: Enable support for ti-cpufreq

AM335x, AM437x, DRA7xx, and AM57xx platforms all now depend on
ti-cpufreq driver to enable proper OPPs for use with cpufreq, so
enable the same.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>