mtd: spi-nor: Add stacked memories support in spi-nor

Each flash that is connected in stacked mode should have a separate
parameter structure. So, the flash parameter member(*params) of the spi_nor
structure is changed to an array (*params[2]). The array is used to store
the parameters of each flash connected in stacked configuration.

The current implementation assumes that a maximum of two flashes are
connected in stacked mode and both the flashes are of same make but can
differ in sizes. So, except the sizes all other flash parameters of both
the flashes are identical.

SPI-NOR is not aware of the chip_select values, for any incoming request
SPI-NOR will decide the flash index with the help of individual flash size
and the configuration type (single/stacked). SPI-NOR will pass on the flash
index information to the SPI core & SPI driver by setting the appropriate
bit in nor->spimem->spi->cs_index_mask. For example, if nth bit of
nor->spimem->spi->cs_index_mask is set then the driver would
assert/de-assert spi->chip_slect[n].

Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@amd.com>
Link: https://lore.kernel.org/all/20230119185342.2093323-1-amit.kumar-mahapatra@amd.com/
State: waiting