mtd: spinand: Add support for setting a bus interface

mtd: spinand: Add support for setting a bus interface

Create a bus interface enumeration, currently only containing the
one we support: SSDR, for single SDR, so any operation whose command is
sent over a single data line in SDR mode, ie. any operation matching
1S-XX-XX.

The main spinand_device structure gets a new parameter to store this
enumeration, for now unused. Of course it is set to SSDR during the SSDR
templates initialization to further clarify the state we are in at the
moment.

This member is subject to be used to know in which bus configuration we
and be updated by the core when we switch to faster mode(s).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>