powerpc/km82xx: rework DIP switch detection

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 <[email protected]>
Signed-off-by: Holger Brunck <[email protected]>
Signed-off-by: Valentin Longchamp <[email protected]>
Acked-by: Heiko Schocher <[email protected]>
cc: Wolfgang Denk <[email protected]>
cc: Detlev Zundel <[email protected]>