mtd: intel-dg: implement region enumeration

mtd: intel-dg: implement region enumeration

In intel-dg, there is no access to the spi controller,
the information is extracted from the descriptor region.

CC: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Co-developed-by: Tomas Winkler <tomasw@gmail.com>
Signed-off-by: Tomas Winkler <tomasw@gmail.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Link: https://lore.kernel.org/r/20250617145159.3803852-3-alexander.usyskin@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>