dts: spi: move `overrun-character` from Nordic to base

Move the `overrun-character` property from the common Nordic SPI
binding to the `spi-controller` base binding. This gives users of the
SPI interface a way to query what the default value is at compile-time,
and potentially avoid allocation of large constant buffers.

Signed-off-by: Jordan Yates <jordan@embeint.com>