tests: drivers: pwm: stm32 board for loopback test

Add overlay file for disco_l475_iot1 board to pwm_loopback test.

The file is based on the existing overlays for twr_ke18f and frdm_k64f.

The new configuration uses both a 16-Bit timer with active prescaler
as well as a 32-Bit timer with deactivated prescaler for coverage.

It is assumed that other STM32 boards will behave like the
disco_l475_iot1. Only this board was available for testing.

Signed-off-by: Tilmann Unte <unte@es-augsburg.de>