p1022ds: use weak CFI flash accessors when DIU is enabled

On the Freescale P1022, the DIU and the LBC share address pins, which means
that when the DIU is active (e.g. the console is on the DVI display), NOR flash
cannot be accessed.  So we use the weak accessor function feature of the CFI
flash code to temporarily switch the pin mux from DIU to LBC whenever we want
to read or write flash.  This has a significant performance penalty, but it's
the only way to make it work.

This change allows the 'saveenv' command to work when the video display is
enabled.  Erasing flash and writing to flash (with the 'cp' command) works,
but reading from flash (with the 'md' and 'cp' commands) does not.  Also, while
flash is being written, the video display will be blank.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>