mtd: plat_nand: add platform probe/remove callbacks

Add optional probe and remove callbacks to the plat_nand driver.

Some platforms may require additional setup, such as configuring the
memory controller, before the nand device can be accessed.  This patch
provides an optional callback to handle this setup as well as a callback
to teardown the setup.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Tested-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>