mtd: nand: add driver for diskonchip g4 nand flash

This patch adds a driver for the diskonchip G4 nand flash device.  It is based
on the driver from the linux kernel.

This also includes a separate SPL driver.  A separate SPL driver is used because
the device operates in a different mode (reliable mode) when loading a boot
image, and also because the storage format of the boot image is different from
normal data (pages are stored redundantly).  The SPL driver basically mimics how
a typical IPL reads data from the device.  The special operating mode and
storage format are used to compensate for the fact that the IPL does not contain
the BCH ecc decoding algorithm (due to size constraints).  Although the u-boot
SPL *could* use ecc, it operates like an IPL for the sake of simplicity and
uniformity, since the IPL and SPL share the task of loading the u-boot image.
As a side benefit, the SPL driver is very small.

[port from linux kernel 3.4 commit 570469f3bde7f71cc1ece07a18d54a05b6a8775d]

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>