ARM: dts: imx7: correct enet ipg clock

ENET "ipg" clock should be IMX7D_ENETx_IPG_ROOT_CLK
rather than IMX7D_ENET_AXI_ROOT_CLK which is for ENET bus
clock.

Based on Andy Duan's patch from the NXP kernel tree.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>