ARM: dts: am335x-boneblack: Enable 1GHz OPP for cpu

Although all PG2.0 silicon may not support 1GHz OPP for the MPU, older
Beaglebone Blacks may have PG2.0 silicon populated and these particular
parts are guaranteed to support the OPP, so enable it for PG2.0 on
am335x-boneblack only.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
eviewed-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>