ppc, mgcoge: add DIP switch detection

This reads the DIP switch on mgcoge. The DIP switch is connected to
the BFTICU (0x40000089) FPGA. If the DIP switch is set the environment
variable 'actual_bank' is set to 0 and starts the SW in bank0.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>