mtd: spi-nor: Introduce the concept of bank

SPI NOR chips are made of pages, which gathered in small groups make
(erase) sectors. Sectors, gathered together, make banks inside the
chip. Until now, there was only one bank per device supported, but we
are about to introduce support for new chips featuring several banks (up
to 4 so far) where different operations may happen in parallel.

Let's allow describing these additional bank parameters, and let's do
this independently of any other value (like the number of sectors) with
an absolute value.

By default we consider that all chips have a single bank.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20230328154105.448540-2-miquel.raynal@bootlin.com
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>