Convert CONFIG_SYS_I2C_SPEED et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_I2C_SPEED
   CONFIG_SYS_I2C_SLAVE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ds414_defconfig b/configs/ds414_defconfig
index ee940ca..90dd10c 100644
--- a/configs/ds414_defconfig
+++ b/configs/ds414_defconfig
@@ -49,6 +49,7 @@
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
+CONFIG_SYS_I2C_SLAVE=0x0
 # CONFIG_MMC is not set
 CONFIG_MTD=y
 CONFIG_SPI_FLASH_STMICRO=y