clk: imx6: don't restrict LDB mux changes on QuadPlus

The LDB mux/gate layout has been fixed on QuadPlus, so there is no need
to restrict the LDB mux changes on this hardware, as the erratum
preventing this from working properly is gone.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>