arm64: dts: msm8996: Transition smp2p and smd to mailbox

The smd and smp2p drivers now support accessing the APCS GLOBAL IPC
register through the mailbox framework, so migrate the msm8996 dts to
use this and remove the syscon based APCS node.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>