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/db-88f6720_defconfig b/configs/db-88f6720_defconfig
index 3ad8b3f..4ac2109 100644
--- a/configs/db-88f6720_defconfig
+++ b/configs/db-88f6720_defconfig
@@ -48,6 +48,7 @@
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
+CONFIG_SYS_I2C_SLAVE=0x0
 CONFIG_MISC=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH_MACRONIX=y