drivers: tee: sandbox: add rpc test ta emulation

This adds support for RPC test trusted application emulation, which
permits to test reverse RPC calls to TEE supplicant. Currently it covers
requests to the I2C bus from TEE.

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>