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-88f6820-gp_defconfig b/configs/db-88f6820-gp_defconfig
index 1580802..f9f1fb8 100644
--- a/configs/db-88f6820-gp_defconfig
+++ b/configs/db-88f6820-gp_defconfig
@@ -51,6 +51,7 @@
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
+CONFIG_SYS_I2C_SLAVE=0x0
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_SDMA=y
 CONFIG_MMC_SDHCI_MV=y