arm64: dts: imx8: conn: fix enet clock setting

enet_clk_ref actually is sourced from internal gpr clocks
which needs a default rate. Also update enet lpcg clock
output names to be more straightforward.

Cc: Abel Vesa <abel.vesa@nxp.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>