boards stm32_min_dev: modify board led pin assignment

Modify board stm32_min_dev aka "blue pill" led pin assignment in dts
from PB12 to PC13.
This board has a user led connected to PC13.
Sample blinky and similar would run out-of-box without additional
circuit to readout PB12 level.
Tested blinky on corresponding board.

Signed-off-by: Stephen Yi <stephen.jin.yee@gmail.com>
1 file changed