boards: arm: gd32f450i_eval: enable pwm support

Enable timer1 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 PB10 (TIMER1_CH2) to PE2
(LED1) to run quick samples such as samples/basic/blinky_pwm.

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