mtd: nand: am335x: spl: Fix copying of image

When offset is not aligned to page address, it is possible that extra offset
will be read from nand. Adjust the image such that first byte of the image
is at load address after the first page is read.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>