cfi_flash: Add weak default for cfi_flash_bank_addr()

cfi_flash_bank_addr(int bank_nr) returns the base addresses of the
requested bank. Introducing this weak default enables boards to override
this functions with a board specific version when required.

This feature will be used in the lwmon5 board update, supporting runtime
detection of 2 board revisions with different flash layouts.

Signed-off-by: Stefan Roese <sr@denx.de>