arm_ffa: introduce sandbox FF-A support

Emulate Secure World's FF-A ABIs and allow testing U-Boot FF-A support

Features of the sandbox FF-A support:

- Introduce an FF-A emulator
- Introduce an FF-A device driver for FF-A comms with emulated Secure World
- Provides test methods allowing to read the status of the inspected ABIs

The sandbox FF-A emulator supports only 64-bit direct messaging.

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/arch/sandbox/dts/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index f0ee0b3..8aaf911 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -451,6 +451,15 @@
 	thermal {
 		compatible = "sandbox,thermal";
 	};
+
+	arm-ffa-emul {
+		compatible = "sandbox,arm-ffa-emul";
+
+		sandbox-arm-ffa {
+				compatible = "sandbox,arm-ffa";
+		};
+	};
+
 };
 
 &cros_ec {