mtd: gpmi-nand bugfix: reset the BCH module when it is not MX23

In MX28, if we do not reset the BCH module. The BCH module may
becomes unstable when the board reboots for several thousands times.
This bug has been catched in customer's production.

The patch adds some comments (some from Wolfram Sang), and fixes it now.

Also change gpmi_reset_block() to static.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org [3.1+]