board: stm32_min_dev: Fix LED0 connection inversion

The on-board LED of the blue and black variants of these boards have LED
logic inverted. This was never observed as most of the time, the LED was
used as a blinky. Fix this by setting its DT bindings to active low.

Signed-off-by: Siddharth Chandrasekaran <siddharth@embedjournal.com>