mtd: rawnand: Add a helper for calculating a page index

For LUN crossing boundaries, it is handy to know what is the index of
the last page in a LUN. This helper will soon be reused. At the same
time I rename page_per_lun to ppl in the calling function to clarify the
lines.

Cc: stable@vger.kernel.org # v6.7
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240223115545.354541-3-miquel.raynal@bootlin.com