drivers: spi: spi_nrfx_spi(m|s): move ORC to DT

Specify the overrun character in Devicetree. Since 0xFF is the most
common value, DT property contains such default.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

tests/drivers/spi/spi_loopback/boards/nrf52840dk_nrf52840.overlay

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2022 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&spi3 {
+	overrun-character = <0x00>;
+};