powerpc/km82xx: rework DIP switch detection

Introduce a struct for the BFTICU FPGA to increase the readability of
the code. And the define CONFIG_SYS_BFTICU_BASE was removed because
the CONFIG_SYS_FPGA_BASE is already the base value for BFTICU registers.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>