sh: Use the plat_nand default partition parser

Use the default partition parser, cmdlinepart, provided by the plat_nand driver.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>