dts: bindings: Add separate binding for Nordic nRF Family SPI Slave

Use separate bindings for nRF Family SPI Slaves and SPI Masters so that
the properties "csn" and "def-char" can be made required for Slaves
(for Masters such settings are not applicable), and to avoid confusion
between the properties "csn" and "cs-gpios" for Master nodes.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>