arm64: dts: add SRAM, MHU mailbox and SCPI support on Juno

This patch adds support for the MHU mailbox peripheral used on Juno by
application processors to communicate with remote SCP handling most of
the CPU/system power management. It also adds the SRAM reserving the
shared memory and SCPI message protocol using that shared memory.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Jon Medhurst (Tixy) <tixy@linaro.org>