runners: bossac: Add speed argument

The current stty command uses a hard code value of 1200.  This is not
compliant with SAM-BA specs and may create compatibility problems.  Add
an optional speed argument with 115200 as default value following SAM-BA
specifications.  All boards that needs a different speed should define
board_runner_args(bossac "--speed=<value>") with value as required
speed.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>