dts: bindings: Add binding for ADXL372 using SPI

This patch adds dts/bindings/adi,adxl372-spi.yaml for
using ADXL372 on SPI bus instead of I2C which is used
in dts/bindings/adi,adxl372.yaml.

Signed-off-by: Jan Tore Guggedal <jantore.guggedal@nordicsemi.no>