Add generic onenand support when connected to GPMC and make the
boards to use it.

The patch has been modified to make it more generic to support all
the boards with GPMC. The patch also remove unused prototype for
omap2_onenand_rephase(void).

Note that board-apollon.c is currently using the MTD_ONENAND_GENERIC
and setting the GPMC timings in the bootloader. Setting the GPMC
timings in the bootloader will not allow supporting frequency
scaling for the onenand source clock.
Signed-off-by: Tony Lindgren <tony@atomide.com>