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/include/configs/maxbcm.h b/include/configs/maxbcm.h
index 0c2185c..c456921 100644
--- a/include/configs/maxbcm.h
+++ b/include/configs/maxbcm.h
@@ -18,7 +18,7 @@
 #define CONFIG_SYS_TCLK		250000000	/* 250MHz */
 
 /* I2C */
-#define CONFIG_SYS_I2C
+#define CONFIG_SYS_I2C_LEGACY
 #define CONFIG_SYS_I2C_MVTWSI
 #define CONFIG_I2C_MVTWSI_BASE0		MVEBU_TWSI_BASE
 #define CONFIG_SYS_I2C_SLAVE		0x0