mtd: rawnand: davinci: store the core chipselect number in platform data

We have the 'ti,davinci-chipselect' property in the device tree, but
when using platform data the driver silently uses the id field of
struct platform_device as the chipselect. This is confusing and we
almost broke the nand support again recently after converting the
platform to common clock framework (which changed the device id in the
clock lookup - the problem is gone now that we no longer acquire the
clock in the nand driver.

This patch adds a new field - core_chipsel - to the platform_data.
Subsequent patches will convert the platforms to using this new field.

Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>