sandbox: i2c: Move platdata structs to header files

At present the structs used by these drivers are declared in the C files
and so are not accessible to dtoc. Move them to header files, as required.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/i2c.h b/arch/sandbox/include/asm/i2c.h
index b482be4..4fc190b 100644
--- a/arch/sandbox/include/asm/i2c.h
+++ b/arch/sandbox/include/asm/i2c.h
@@ -11,4 +11,19 @@
 	bool test_mode;
 };
 
+/**
+ * struct i2c_emul_uc_plat - information about the emulator for this device
+ *
+ * This is used by devices in UCLASS_I2C_EMUL to record information about the
+ * device being emulated. It is accessible with dev_get_uclass_plat()
+ *
+ * @dev: Device being emulated
+ * @idx: of-platdata index, set up by the device's bind() method if of-platdata
+ *	is in use
+ */
+struct i2c_emul_uc_plat {
+	struct udevice *dev;
+	int idx;
+};
+
 #endif /* __asn_i2c_h */