spi: gpio: Remove stale documentation part

The SPI GPIO driver doesn't support hard coded absolute GPIO numbers
anymore. However, it may still be instantiated from board files with
help of GPIO lookup tables or device properties. Neither of this is
covered by the old part of the documentation, it's the opposite, i.e.
old documentation pretend that antique approach still works. With all
this said, remove stale and confusing part of the documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250205132127.742750-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>