nand: remove CONFIG_SYS_NAND_PAGE_SIZE

We only need to read in the size of struct image_header and thus don't
need to know the page size of the nand device.

Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>