spi: docs: improve the SPI userspace API documentation

spi: docs: improve the SPI userspace API documentation

This doc is fairly outdated and only uses legacy device instantiation
terminology. Let us update it and also mention the OF and ACPI device
tables, to make easier for users to figure out how should be defined.

Also, mention that devices bind could be done in user-space now using
the "driver_override" sysfs entry.

Suggested-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211119121139.2412761-1-javierm@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>