i2c: Rename CONFIG_SYS_I2C to CONFIG_SYS_I2C_LEGACY

It is quite confusing that CONFIG_SYS_I2C selects the legacy I2C and
CONFIG_DM_I2C selects the current I2C. The deadline to migrate I2C is less
than a year away.

Also we want to have a CONFIG_I2C for U-Boot proper just like we have
CONFIG_SPL_I2C for SPL, so we can simplify the Makefile rules.

Rename this symbol so it is clear it is going away.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/arch/arm/mach-kirkwood/include/mach/config.h b/arch/arm/mach-kirkwood/include/mach/config.h
index f5538f4..a4b5630 100644
--- a/arch/arm/mach-kirkwood/include/mach/config.h
+++ b/arch/arm/mach-kirkwood/include/mach/config.h
@@ -96,7 +96,7 @@
  */
 #if defined(CONFIG_CMD_I2C) && !CONFIG_IS_ENABLED(DM_I2C)
 #ifndef CONFIG_SYS_I2C_SOFT
-#define CONFIG_SYS_I2C
+#define CONFIG_SYS_I2C_LEGACY
 #define CONFIG_SYS_I2C_MVTWSI
 #endif
 #define CONFIG_SYS_I2C_SLAVE		0x0