i2c: add FUNC flag for slave capabilities

So users can check in advance if there is slave support.
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>

include/uapi/linux/i2c.h

@@ -87,6 +87,7 @@ struct i2c_msg {
 #define I2C_FUNC_PROTOCOL_MANGLING	0x00000004 /* I2C_M_IGNORE_NAK etc. */
 #define I2C_FUNC_SMBUS_PEC		0x00000008
 #define I2C_FUNC_NOSTART		0x00000010 /* I2C_M_NOSTART */
+#define I2C_FUNC_SLAVE			0x00000020
 #define I2C_FUNC_SMBUS_BLOCK_PROC_CALL	0x00008000 /* SMBus 2.0 */
 #define I2C_FUNC_SMBUS_QUICK		0x00010000
 #define I2C_FUNC_SMBUS_READ_BYTE	0x00020000