cfi_flash: Add interface for flash verbosity control

Add interface for flash verbosity control. It allows
to disable output from low-level flash API. It is useful
when calling these low-level functions from context other
than flash commands (for example the MTD/CFI interface
implmentation).

Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-by: Stefan Roese <sr@denx.de>