clk: imx: correct pwm7 clock name in driver for i.MX6UL

Don't capitalize p in the pwm7 clock name.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>