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/pinecube_defconfig b/configs/pinecube_defconfig
index b2a8370..742da36 100644
--- a/configs/pinecube_defconfig
+++ b/configs/pinecube_defconfig
@@ -11,6 +11,8 @@
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
+CONFIG_SYS_I2C_SLAVE=0x7f
+CONFIG_SYS_I2C_SPEED=400000
 # CONFIG_NETDEVICES is not set
 CONFIG_AXP209_POWER=y
 CONFIG_AXP_DCDC2_VOLT=1250