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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>