Update the ts72xx platform's nand driver support.

This changes the ts72xx platform from using a custom nand driver
(ts7250.c) to the generic platform nand driver (plat_nand.c).

Tested on TS-7250 with 32MiB NAND.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Tested-by: Matthieu Crapet <mcrapet@gmail.com>
Cc: Jesse Off <joff@embeddedARM.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>