nand_util.c: Correct licensing

nand_util.c: Correct licensing

Prior to SPDX licensing this file was GPL-2.0 with Freescale granting
rights for "or later" for their contributed code.  We incorrectly moved
this file to GPL-2.0+, so correct it to GPL-2.0.  In addition we cannot
easily denote in the file where or what code is "or later", so just set
that aside for now and the file as a whole is GPL-2.0 regardless.

Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Tom Rini <trini@ti.com>