mtd: cfi: change CONFIG_SYS_MAX_FLASH_BANKS_DETECT as boolean

Prepare migration to Kconfig.

CONFIG_SYS_MAX_FLASH_BANKS_DETECT becomes boolean and
CONFIG_SYS_MAX_FLASH_BANKS define the MAX size, also used
for detection when CONFIG_SYS_MAX_FLASH_BANKS_DETECT=y
(CFI_MAX_FLASH_BANKS = CONFIG_SYS_MAX_FLASH_BANKS).

CONFIG_SYS_MAX_FLASH_BANKS become mandatory when
CONFIG_SYS_MAX_FLASH_BANKS_DETECT is activated.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Stefan Roese <sr@denx.de>