boards: arm: stm32f746g_disco: use DT for PWM pinmux configuration

Use the new DT facilities to configure PWM pinmux on
STM32F746G-DISCOVERY board.

NOTE: DT entry for timers1/pwm1 has been replaced by timers3/pwm3 as it
was the pwm referenced in the pinmux file.

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