cfi_flash: Simplify dynamic flash bank number detection

This patch simplifies the use of CONFIG_SYS_MAX_FLASH_BANKS_DETECT. By
moving these optional variables and defines into the common code, board
specific code is minimized. Currently only the following board use
this feature:

APC405, IDS8247, TQM834x

And IDS8247 doesn't seem to really need this feature, since its not
updating the bank number variable at all. So this patch removes the
definition of CONFIG_SYS_MAX_FLASH_BANKS_DETECT from this board port.

This new framework will be used by the upcoming lwmon5 update as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>