Software executing in the normal world and in the trusted world at
exception levels lower than EL3 will request runtime services using the
SMC instruction.

See the documentation here:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/
rt-svc-writers-guide.md

This to be implemented as an EL3 Runtime Service in rockchip BL31
platform port, using the "SiP Service Call" range as specified in the
SMC Calling Convention.
This doesn't support any SMC yet, we will support it in later.

Change-Id: I0a638dd0b653c28b08f79d89f77ed7c69864017d