siemens: change LED indication in DFU mode

In order to have the same LED indication like in another product
when ready for updating, enable only red led and disable status
LED when entering DFU mode.

The status LED is only switched off when defined in board file.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Wolfgang Denk <wd@denx.de>