mtd: spi-nor: Decouple SPI NOR's device_node from controller device

mtd: spi-nor: Decouple SPI NOR's device_node from controller device

The problem this patch is trying to address is such, that SPI NOR flash
devices attached to a dedicated SPI NOR controller cannot read their
properties from the associated struct device_node.

A couple of facts first:
1) Each SPI NOR flash has a struct spi_nor associated with it.
2) Each SPI NOR flash has certain device properties associated
   with it, for example the OF property 'm25p,fast-read' is a
   good pick. These properties are used by the SPI NOR core to
   select which opcodes are sent to such SPI NOR flash. These
   properties are coming from spi_nor .dev->of_node .

The problem is, that for SPI NOR controllers, the struct spi_nor .dev
element points to the struct device of the SPI NOR controller, not the
SPI NOR flash. Therefore, the associated dev->of_node also is the
one of the controller and therefore the SPI NOR core code is trying to
parse the SPI NOR controller's properties, not the properties of the
SPI NOR flash.

Note: The m25p80 driver is not affected, because the controller and
      the flash are the same device, so the associated device_node
      of the controller and the flash are the same.

This patch adjusts the SPI NOR core such that the device_node is not
picked from spi_nor .dev directly, but from a new separate spi_nor
.flash_node element. This let's the SPI NOR controller drivers set up
a different spi_nor .flash_node element for each SPI NOR flash.

This patch also fixes the controller drivers to be compatible with
this modification and correctly set the spi_nor .flash_node element.

This patch is inspired by 5844feeaa4154d1c46d3462c7a4653d22356d8b4
mtd: nand: add common DT init code

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>