spi: cadence-qspi: Add Renesas RZ/N1 support

Merge series from "Miquel Raynal (Schneider Electric)" <miquel.raynal@bootlin.com>:

This series adds support for the QSPI controller available on Renesas
RZ/N1S and RZ/N1D SoC. It has been tested with a custom board (see last
SPI patch for details), but has been tested by Wolfram (thank you!) on
the DB board.
Link: https://lore.kernel.org/linux-devicetree/20260116114852.52948-2-wsa+renesas@sang-engineering.com/
Adding support for this SoC required a few adaptations in the Cadence
QSPI driver. The bulk of the work is in the few last patches. Everything
else is just misc style fixes and improvements which bothered me while I
was wandering.

In order to support all constraints, I sometimes used a new quirk (for
the write protection feature and the "no indirect mode"), and sometimes
used the compatible directly. The ones I thought might not be RZ/N1
specific have been implemented under the form of a quirk, in order to
ease their reuse. The other adaptations, which I believe are more
Renesas specific, have been handled using the compatible. This is all
very arbitrary, and can be discussed.