cti_flash.c: use __weak when requested for flash_(read|write){8,16,32,64}

For various reasons (design, errata) boards may need to implement their
own versions of these accessors.  So in the case of
CONFIG_CFI_FLASH_USE_WEAK_ACCESSOR mark the functions as weak.  In the
normal case mark them as static to allow for better optimization.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
[trini: Reword commit message]
Signed-off-by: Tom Rini <trini@ti.com>