cfi_flash: Make all flash access functions weak

This patch defines all flash access functions as weak so that
they can be overridden by board specific versions.

This will be used by the upcoming VCTH board support where the NOR
FLASH unfortunately can't be accessed memory-mapped. Special
accessor functions are needed here.

To enable this weak functions you need to define
CONFIG_CFI_FLASH_USE_WEAK_ACCESSORS in your board config header.
Otherwise the "old" default functions will be used resulting
in smaller code.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>