dts: bindings: spi: Fix "nordic,nrf-spis" binding

The nRF SPIS peripheral is not an SPI controller but an SPI target
device, so its binding should not include "spi-controller.yaml"
(and so far it did, through "nordic,nrf-spi-common.yaml").
Fix this so that it is no longer possible to incorrectly add child
DT nodes as "devices" on the SPIS "bus" and then for example misuse
the SPI_CONFIG_DT() macro for such constructs.

Since this requires a small correction in the "overrun-character"
property in "nordic,nrf-spi-common.yaml", improve also its description
by adding there information from the corresponding property from
"spi-controller.yaml".

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