arm64: dts: qcom: sdm845: add SLPI FastRPC support

Qualcomm SDM845 SoC features a SLPI DSP which uses FastRPC through
an allocated memory region to load files from the host filesystem
such as sensor configuration files.

Add a FastRPC node at /dev/fastrpc-sdsp and a DMA region, similar to
downstream, to allow userspace to communicate with the SLPI via the
FastRPC interface for initializing the sensors on the SLPI.

Signed-off-by: Dylan Van Assche <me@dylanvanassche.be>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230406173148.28309-4-me@dylanvanassche.be