mtd: plat_nand: allow platform to set partitions

Add optional callback to allow platform to initialize partitions.

Static partitions on a nand device could vary depending on the size of the
device.  This patch allows an optional platform callback to be used to
setup this partition information at runtime.

Scan order is:
	1) chip.part_probe_types
	2) chip.set_parts
	3) chip.partitions
	4) full mtd device (fallback for no partitions)

Some of the existing nand drivers could possibly be replaced by the
plat_nand driver by using this patch.  These include autcpu12.c and
ts7250.c drivers.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
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>