[MTD] [NAND] at91_nand: Make mtdparts option can override board info

Call parse_mtd_partitions before checking board's partition_info, so
that "mtdparts=" option can override board's default setting.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>