mtd: rawnand: arasan: Rename the data interface register

There are 2 timing registers:
- "data interface"
- "timings"

So far, the "data interface" register was named "timings" which begins
misleading when bringing support for the "timings" register. Rename it
to "data_iface".

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210527084959.208804-1-miquel.raynal@bootlin.com