ARM: dts: omap: Drop references to opp.txt

opp.txt is getting removed with the OPP binding converted to DT schema.
As it is unusual to reference a binding doc from a dts file, let's just
remove the reference.

Cc: "Benoît Cousson" <bcousson@baylibre.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>