Convert CONFIG_SYS_OMAP24_I2C_SLAVE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_OMAP24_I2C_SLAVE
   CONFIG_SYS_OMAP24_I2C_SPEED

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Paul Kocialkowski <contact@paulk.fr>
diff --git a/configs/twister_defconfig b/configs/twister_defconfig
index 03db9df..f49c234 100644
--- a/configs/twister_defconfig
+++ b/configs/twister_defconfig
@@ -31,6 +31,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=omap2-nand.0:512k(MLO),1m(u-boot),256k(env1),256k(env2),6m(kernel),-(rootfs)"
 CONFIG_CMD_UBI=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_SYS_OMAP24_I2C_SPEED=400000
 CONFIG_MMC_OMAP_HS=y
 CONFIG_NAND=y
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y