cfi_flash: Add prototypes of overridable functions

This commit adds some prototypes into include/mtd/cfi_flash.h.
These functions are defined with a weak attribute in
drivers/mtd/cfi_flash.c.
This means they can be overrided by board-specific ones
if necessary.

When defining such functions under board/ directory or
somewhere, cfi_flash.h should be included.
This makes sure that board-specfic cfi functions
are defined in a correct prototype.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Stefan Roese <sr@denx.de>