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/sniper_defconfig b/configs/sniper_defconfig
index a3bae56..84ba175 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -35,6 +35,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_ISO_PARTITION=y
 CONFIG_EFI_PARTITION=y
+CONFIG_SYS_OMAP24_I2C_SPEED=400000
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y