km_arm: enable POST for these boards

The current km_arm boards have a Power-On test jumper. When this
jumper is set, this triggers some Power-On tests on the board.

This patch enables the support of this jumper for starting the
memory_regions test when the jumper is set.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>