dt-bindings: soc: qcom: Introduce GLINK SPI bindings

The glink spi transport uses the wcd-spi driver to send packets to and
from a remote proc over a spi bus. The FIFO used on the spi bus are
described by four registers, add bindings for these properties.

Change-Id: Ifcc6ed881a9b3a23763b9fb1678062f1afcd2f42
Signed-off-by: Chris Lew <clew@codeaurora.org>