Convert CONFIG_SYS_I2C_LEGACY to Kconfig and add CONFIG_[ST]PL_SYS_I2C_LEGACY

First, we convert CONFIG_SYS_I2C_LEGACY to Kconfig.  Next, as you cannot
have SYS_I2C_LEGACY and DM_I2C at the same time, introduce
CONFIG_SPL_SYS_I2C_LEGACY so that we can enable the legacy option only
in SPL.  Finally, for some PowerPC cases we also need
CONFIG_TPL_SYS_I2C_LEGACY support.  Convert all of the existing users to
one or more symbols.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/riotboard_defconfig b/configs/riotboard_defconfig
index 2f45536..e0c20c8 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -40,6 +40,8 @@
 CONFIG_SYS_MMC_ENV_DEV=2
 CONFIG_DM=y
 CONFIG_BOUNCE_BUFFER=y
+CONFIG_SYS_I2C_LEGACY=y
+CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_FSL_USDHC=y
 CONFIG_DM_SPI_FLASH=y