dt-bindings: pwm: fsl-ftm: Add compatible string for i.MX8QM

i.MX8QM integrates a new version of FTM IP block. It adds eight PWM
enable bits in FTM_SC register.

Add a new compatible string of "fsl,imx8qm-ftm-pwm" for i.MX8QM to
differentiate it from the previous SoCs.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>