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/am335x_baltos_defconfig b/configs/am335x_baltos_defconfig
index 548e045..97e6001 100644
--- a/configs/am335x_baltos_defconfig
+++ b/configs/am335x_baltos_defconfig
@@ -47,6 +47,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=omap2-nand.0:128k(SPL),128k(SPL.backup1),128k(SPL.backup2),128k(SPL.backup3),1920k(u-boot),-(UBI)"
 CONFIG_CMD_UBI=y
 CONFIG_ISO_PARTITION=y
+CONFIG_SYS_OMAP24_I2C_SPEED=1000
 CONFIG_MMC_OMAP_HS=y
 CONFIG_NAND=y
 CONFIG_PHYLIB=y