cpufreq: OMAP: donot allow to be used with device tree

We now use Soc generic cpufreq-cpu0 driver using DT entries.
However to allow cpufreq-cpu0 and omap-cpufreq drivers to co-exist,
we need to ensure the following using the same image:
1. With device tree boot, we use cpufreq-cpu0
2. With non device tree boot, we use omap-cpufreq

In the case of (1), we will have cpu OPPs and regulator registered
as part of the DT, to ensure that omap-cpufreq and cpufreq-cpu0 dont
conflict in managing the frequency of the same cpu, we should not
permit init to pass in omap-cpufreq.

In the case of (2), we will not have the cpufreq-cpu0 device, hence
only omap-cpufreq will be active.

So, to acommodate both these usage conditions, we fail init of
omap-cpufreq when we boot with device tree.

Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: "Benoît Cousson" <b-cousson@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Keerthy <j-keerthy@ti.com>
Cc: linux-omap@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: cpufreq@vger.kernel.org
Cc: linux-pm@vger.kernel.org

Signed-off-by: Nishanth Menon <nm@ti.com>