boards: arm: gd32f403z_eval: enable pwm support

Enable timer0 and its pwm node. The board does not directly have any LED
connected to a PWM output. However, following the official
"TIMER_Breath_LED" example, user can bridge PA8 (TIMER0_CH0) to PF0
(LED2) to run quick samples such as samples/basic/blinky_pwm.

Note that a prescaler value of 256 has been chosen so that basic PWM
samples (blinky_pwm and fade_led) can be run. This value may be
overriden in specific samples if needed.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>