clk: imx: scu: add enet rgmii gpr clocks

enet tx clk actually is sourced from a gpr divider, not default enet
clk. Add enet grp clocks for user to use correctly.

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