arm64: dts: msm8996: Add modem remoteproc

Add the modem remoteproc node and the child smd-edge in order to be able
to boot the modem Hexagon found in MSM8996 based devices.

Also extend the tcsr mutex node size, to cover the registers at the end
of the block, used for halting the modem subsystem.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>