samples: drivers: pwm led running on stm32 boards

On the stm32l073rz nucleo and stm32f091rc nucleo
boards, the DTS pwm-leds node is disabled, because
the output pin might conflict with SPI1.
It is necessary to enable the node in the overlay
to compile and run the sample.

Signed-off-by: Francois Ramu <francois.ramu@st.com>