Documentation: devicetree: m25p80: add "nor-jedec" binding

Almost all flash that are "compatible" with m25p80 support the JEDEC
READ ID opcode (0x9F), and in fact, that is often the only thing that is
used to differentiate them. Let's add a compatible string that
represents this lowest common denominator of compatibility.

Device trees can still specify manufacturer/device names in addition,
but (until some reason is found to differentiate between them through
device tree) software will likely want to bind just against the generic
name, and avoid unnecessarily growing its device ID binding tables.

This is related to the work of commit a5b7616c ("mtd:
m25p80,spi-nor: Fix module aliases for m25p80"), which showed that
maintaining these device tables as stable device-tree/modalias binding
tables is not a worthwhile burden for mostly-comptatible flash.

At the same time, let's update the binding doc to point to the
m25p_ids[] ID list instead of spi_nor_ids[]. The former can be used for
device tree bindings, but the latter cannot. In the future, we should
pare down the m25p_ids[] list to only those IDs which are actually used
in device trees.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>