docs: spi: add documentation for userspace device instantiation

Document the new_device and delete_device sysfs attributes on SPI
controllers:

  - Documentation/spi/instantiating-devices.rst: describes when and
    why this interface is needed, accepted parameters, usage examples,
    and limitations.
  - Documentation/ABI/testing/sysfs-class-spi-master: formal ABI
    entry for both attributes.

Signed-off-by: Vishwaroop A <va@nvidia.com>
Link: https://patch.msgid.link/20260803104614.2548375-3-va@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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