tkey: Add a sandbox driver for real TKey hardware

Add a sandbox TKey driver that enables communication with physical TKey
devices via serial port (/dev/ttyACM0). This allows testing TKey
functionality in sandbox with real hardware.

The driver:
  - Opens the configured device path from device tree
  - Configures TTY parameters using os_tty_set_params()
  - Implements read/write operations for TKey protocol
  - Supports both read() and read_all() operations

Device tree configuration:

  tkey-test {
    compatible = "sandbox,tkey";
    sandbox,device-path = "/dev/ttyACM0";
  };

Series-to: concept
Cover-letter:
tkey: Provide basic support for Tillitis TKey
This device provides a way to sign data using an internal, unique key.
It can be useful for features such as unlocking an encrypted disk.

This series provides basic support for the Tkey, with a uclass, two
sandbox drivers (emulator and serial), a simple command some tests.
END

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/sandbox.dtsi

@@ -356,6 +356,11 @@
 		clock-frequency = <1000000>;
 	};
 
+	tkey-test {
+		compatible = "sandbox,tkey";
+		sandbox,device-path = "/dev/ttyACM0";
+	};
+
 	tpm {
 		compatible = "google,sandbox-tpm";
 	};