m28evk/mx28evk: fix nand_update_full

m28evk/mx28evk: fix nand_update_full

- since commit 418396e212b59bf907dbccad997ff50f7eb61b16 nand write.raw
can take the number of page to be written as an argument. nand_update_full
is passing the size (in bytes) to nand write.raw. This value was previously
ignored but now breaks the write.
- this patch updates the default environment of these boards to provide a
pagecount instead of a size to nand write.raw.
- tested on a mx28evk with a 4k page NAND and on a custom board with a
2k page NAND.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>