drivers: pwm: add a CH32V00x General-prupose Timer Module (GPTM) driver

The GPTM is a general purpose module with a 16 bit prescaler, 16 bit
counter, and 4 compare units that can be used for PWM generation.

Use the same style as gd32 where the timer is a counter and the PWM
mode is a child node.

Signed-off-by: Michael Hope <michaelh@juju.nz>