clk: imx: imx8mn: drop unused pll enum

The PLL enum definition is not used, so drop it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lkml.kernel.org/r/1571122989-29361-1-git-send-email-peng.fan@nxp.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>