* making SPI CS optional
* CS pin should be declared as optional or required in CONFIG_SCHEMA
* changed SPI_DEVICE_SCHEMA to a function, like i2c
* added spi_device_schema() to pcd8544, lint fixes
* updated max31856 with new spi_device_schema()
* cleanup imports
Co-authored-by: Ilya Goldberg <iggie@mac.com>