tbs2910: explicitly set boot address

Set missing boot address in bootm command. This fixes the error:
 Wrong Image Format for bootm command
 ERROR: can't get kernel image!

Reported-by: Uwe Scheffler <scheffler.u@web.de>
Signed-off-by: Soeren Moch <smoch@web.de>
Tested-by: Uwe Scheffler <scheffler.u@web.de>