[MTD] RBTX4939: add MTD support

Add platform support for NOR flash chips on RBTX4939 board.
This board has complex flash mappings, controlled by its DIPSW setting.

[akpm@linux-foundation.org: Use min_t]
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Ralf Bächle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>