optee: isolate smc abi

Isolate the ABI based on raw SMCs. Code specific to the raw SMC ABI is
moved into smc_abi.c. This makes room for other ABIs with a clear
separation.

The driver changes to use module_init()/module_exit() instead of
module_platform_driver(). The platform_driver_register() and
platform_driver_unregister() functions called directly to keep the same
behavior. This is needed because module_platform_driver() is based on
module_driver() which can only be used once in a module.

A function optee_rpc_cmd() is factored out from the function
handle_rpc_func_cmd() to handle the ABI independent part of RPC
processing.

This patch is not supposed to change the driver behavior, it's only a
matter of reorganizing the code.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>