boards: arm: stm32_mini_a15: Mark board as deprecated
boards: arm: stm32_mini_a15: Mark board as deprecated

The original developer no longer has a working board and isn't
interested in maintaining support for the board.  Mark the board
deprecated for now and see if anyone wants to pick it up, otherwise will
remove it in a future release.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

GiulianoFranchetto/zephyr,zephyriot/zephyr,ldts/zephyr,explora26/zephyr,aceofall/zephyr-iotos,explora26/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,punitvara/zephyr,fbsder/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,punitvara/zephyr,mbolivar/zephyr,nashif/zephyr,mbolivar/zephyr,galak/zephyr,nashif/zephyr,fbsder/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,fbsder/zephyr,galak/zephyr,fbsder/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,nashif/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,mbolivar/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr