Convert CONFIG_SYS_I2C_SOFT et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_I2C_SOFT
   CONFIG_SYS_I2C_SOFT_SPEED
   CONFIG_SYS_I2C_SOFT_SLAVE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ethernut5_defconfig b/configs/ethernut5_defconfig
index 4da3c1d..7b58b71 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -55,6 +55,8 @@
 CONFIG_CLK=y
 CONFIG_CLK_AT91=y
 CONFIG_AT91_GPIO=y
+CONFIG_SYS_I2C_SOFT=y
+CONFIG_SYS_I2C_SOFT_SLAVE=0
 CONFIG_MTD=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_MTD_RAW_NAND=y