mtd: cfi: introduce CFI_FLASH_BANKS

Replace CONFIG_SYS_MAX_FLASH_BANKS by CFI_FLASH_BANKS to prepare
Kconfig migration and avoid to redefine CONFIG_SYS_MAX_FLASH_BANKS
in cfi_flash.h.

After this patch CONFIG_SYS_MAX_FLASH_BANKS should be never used in
the cfi code: use CFI_MAX_FLASH_BANKS for struct size or CFI_FLASH_BANKS
for number of CFI banks which can be dynamic.

This patch modify all the files which include mtd/cfi_flash.h.

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