ARM: OMAP3+: use cpu0-cpufreq driver

With OMAP3+ and AM33xx supported SoC having defined CPU DT
entries with operating-points defined, we can now
use the SoC generic cpu0-cpufreq driver to start
using it, lets now switch to the generic driver.

As part of this change, switch the dummy clock node to use
cpufreq-cpu0. This is an suggested solution till we have
OMAP clock nodes in DT. Once the DT conversion is complete,
we can then do: clocks = <&dpll_mpu_ck>; or the SoC specific
equivalent.

Inspired by patch: https://patchwork.kernel.org/patch/2067841/
now made generic.

Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
cpufreq: omap: remove omap-cpufreq

We can now use cpufreq-cpu0 driver using DT entries.
remove the redundant omap-cpufreq driver from the tree.
Remove MAINTAINERS file entry for the same as well.

Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Nishanth Menon <nm@ti.com>