mtd: gpmi: make blockmark swapping optional

With a flash-based BBT there is no reason to move the Factory Bad
Block Marker from the data area buffer (to where it is mapped by the
GPMI NAND controller) to the OOB buffer. Thus, make this feature
configurable via DT. This is required for the Ka-Ro electronics
platforms.

In the original code 'this->swap_block_mark' was synonymous with
'!GPMI_IS_MX23()', so use the latter at the relevant places.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>