clk: imx6q: Add definition for IMX6QDL_CLK_PWM1

Enabling CONFIG_CMD_PWM, and consequently CONFIG_DM_PWM, causes the pwm
command to fail due to a missing clock.

    => pwm enable 0 0
    Enable clock-controller@20c4000 failed
    Failed to enable per_clk
    pwm: '0' not found
    Command 'pwm' failed: Error -2

Note that it is trivial to also add the clocks for PWM2, PWM3, and PWM4.
However, I only tested PWM1 and thus only added the clock for that
signal.

Signed-off-by: Emil Kronborg <emil.kronborg@protonmail.com>