pwm: Add Renesas RZ/G2L MTU3a PWM driver

The RZ/G2L Multi-Function Timer Pulse Unit 3 (a.k.a MTU3a) uses
one counter and two match components to configure duty_cycle
and period to generate PWM output waveform.

Add basic support for RZ/G2L MTU3a PWM driver by creating separate
PWM channels for each IOs.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>