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/bananapi_m1_plus_defconfig b/configs/bananapi_m1_plus_defconfig
index 1f5a51b..2f0c22f 100644
--- a/configs/bananapi_m1_plus_defconfig
+++ b/configs/bananapi_m1_plus_defconfig
@@ -13,6 +13,8 @@
 CONFIG_NETCONSOLE=y
 CONFIG_SCSI_AHCI=y
 CONFIG_SYS_I2C_MVTWSI=y
+CONFIG_SYS_I2C_SLAVE=0x7f
+CONFIG_SYS_I2C_SPEED=400000
 CONFIG_PHY_REALTEK=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_RGMII=y