runners: add '--dt-flash=y' for silabs commander

Due to a bug in scripts/west_commands/runners/core.py, the value of
--dt-flash can currently be True, False, or the default value, 'n'. The
script silabs_commander.py only check the truth value of --dt-flash,
 so the default value is interpreted as a truthy. That means that the
default case was effectively the same as if dt-flash was set to True.

Set this flag to True by default explicitly, in the same
way as done for pyocd or jlink.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>