M5329EVB, M5373EVB: Remove CONFIG_NANDFLASH_SIZE

In the case of M5373EVB we always had NANDFLASH_SIZE=16, so just use it
directly.  In the case of M5329EVB we had not removed the rest of NAND
support when saying we didn't have NAND, so instead use that to key off
of rather than NANDFLASH_SIZE.

Cc: TsiChung Liew <Tsi-Chung.Liew@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>