mtd: physmap_of: add a hook for Gemini flash probing

In order to support device tree probing of Gemini NOR flash
chips, a certain register in the syscon needs to be poked
to enable parallel flash mode.

Such things used to happen in "necessarily different" board
file code, and this indeed was also done for the Gemini, so
the MTD driver could treat it as any memory-mapped NOR flash,
but this is not the way in the future: board files need to
go, and hardware concerns distributed down to the applicable
drivers.

This adds a hook in the same way that the Versatile did: if
the Kconfig symbol is not selected the net total of supporting
Gemini should be zero bytes of added code. To live up to this
promise, also the return value error print from the Versatile
extra probe call get to be removed in this patch, all printing
need to happen in the add-ons.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>