memory: aemif: add support for board files

Currently aemif is supported in two places separately. By the platform
driver in drivers/memory and by a hand crafted driver in mach-davinci.

We want to drop the latter but also keep the legacy mode. Add support
for board files to the aemif driver.

The new structure in platform data currently only contains the chip
select number, since currently existing users don't require anything
else, but it can be extended in the future.

While extending the platform data struct, add kernel docs describing
its members.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>