clk: imx: correct uart4_serial clock name in driver for i.MX6UL

Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>