siemens: add led cmd for flexible LED control

* remove setting LED in user button function.
   We want to decouple reading user button and setting LED. This
   two things need to be done independently.

 * led cmd can be used to control LEDs that are defined in board file
   having a led cmd, one can easily set LEDs in u-boot shell. For
   example bootcmd can be extended to disable status LED before
   loading kernel.

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>