mtd: physmap_of: add a hook for Intel IXP4xx flash probing

In order to support device tree probing of IXP4xx NOR flash
chips, a certain big-endian or mixed-endian memory access
pattern need to be used.

I have opted to use the pattern set by previous plug-ins
to physmap for Gemini and Versatile, just override some
functions and reuse most of the physmap core code as it
is to minimize maintenance.

Parts of drivers/mtd/ixp4xx.c are copied into this file.

After we have IXP4xx converted fully to device tree, the
drivers/mtd/ixp4xx.c file will be deleted and this will
be the only access pattern to the IXP4xx flash.

I did not keep the quirk in the flash write function
after probe, where the old code for a while checks for
access to odd addresses, fails and assigns a "faster"
write function once it has convinced probe to only use
2-byte accesses. As we mandate that this device should
be using bank-width = <2> this should not be a problem
unless misconfigured.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>