[tee][optee] Implement basic RPC command handling

One of the core RPCs in the OP-TEE spec executes a command specified and
parameterized by an OP-TEE message format stored in memory. This CL
adds support for the commands that allocate/free shared memory and load
trusted applications for the trusted world. Supporting these commands
are enough to allow for simple trusted applications to be loaded and
executed in OP-TEE.

Test: Manual testing on both VIM2 and Astro to verify that the
TEEC_OpenSession API loads and returns successfully for a test "hello
world" trusted application-client application pair.

Change-Id: I1a2baa39c81090c678506e5b76abd792236835dd

[tee][optee] Add Context initialize and finalize

The first building block concept for the TEE Client API is a Context.
A Context is a logical connection which exists between a Client
Application and a TEE. For our implementation, this is nothing more than
opening and closing the TEE device driver. As a precaution, we also
validate that the TEE device we've opened is compliant with
GlobalPlatform.

Test: Manually validated that the tee-test succeeded in opening the
device.
Change-Id: I3ea929f8148069c490b6eba83c28e5fa2808b441

[tee] Add TEE Client API skeleton

The GlobalPlatform TEE Client API provides a communications API for
connecting Client Applications running in a rich execution environment
(in our case, Fuchsia) with an associated Trusted Application running
inside a Trusted Execution Environment (TEE).

This commit is limited to adding the constants, macros, data structures
and function stubs as defined by the TEE Client API specification. All
of the functions will return NOT_IMPLEMENTED for now.

Test: The library successfully builds, but no usages of it yet and its
behaviors are unimplemented.
Change-Id: I2ec9b925882592e98ac82a0d27ba104049f4639f