boards: arm: disco_l475_iot1: use DT for PWM pinmux configuration

Use the new DT facilities to configure PWM pinmux on
DISCO-L475VG-IOT01A board.

NOTE: pinctrl file pointed to a wrong variant l475r(c-e-g)tx, however,
the SoC in the board corresponds to l475v(c-e-g)tx.

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