ARM: dts: dra7: Enable 1.5 GHz operation for the CPU

After ti-opp-supply driver is in place to provide AVS Class0 and abb
regulator scaling support let's enable 1.5GHz for the cpu.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>