ARM: dts: imx7d: use operating-points-v2 for cpu

This patch uses "operating-points-v2" instead of
"operating-points" to be more fit with cpufreq-dt
driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>