dt-bindings: spi: spi-cadence: Add label property

Add a label property to allow a custom name to be used for identifying the
controller on a board. This is useful when there is more than one
controller present. Similar change was done by commit 412b7a52


("dt-bindings: eeprom: at24: Add label property for AT24").

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/82cd1a57397867b5a1039cd15244344c02a3ece1.1691047461.git.michal.simek@amd.com
Signed-off-by: Mark Brown <broonie@kernel.org>