Documentation: devres: add missing SPI helper

Add devm_spi_alloc_master() and devm_spi_alloc_slave() to devres.rst.
They are introduced by
commit 5e844cc3 ("spi: Introduce device-managed SPI controller allocation").
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/20220923141803.75734-1-yangyingliang@huawei.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/driver-model/devres.rst

@@ -429,6 +429,8 @@ SLAVE DMA ENGINE
   devm_acpi_dma_controller_register()
 
 SPI
+  devm_spi_alloc_master()
+  devm_spi_alloc_slave()
   devm_spi_register_master()
 
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