pwm: Add i.MX TPM PWM driver support

i.MX7ULP has TPM(Low Power Timer/Pulse Width Modulation Module) inside,
it can support multiple PWM channels, all the channels share same
counter and period setting, but each channel can configure its duty and
polarity independently.

There are several TPM modules in i.MX7ULP, the number of channels in TPM
modules are different, it can be read from each TPM module's PARAM
register.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>