Fix the NAND size overflow issue.

When the total size of all NAND devices exceeds 4 GiB, the size  will
overflow. This patch tries to fix this.

Note that we still have a problem when a single NAND device is bigger
than 4 GiB: then the overflow would actually happen earlier, i. e.
when storing the size in nand_info[].size, as nand_info[].size is an
"u_int32_t".

Signed-off-by: Jason Jin <Jason.jin@freescale.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>