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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 5d83c26..e4b5d92 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -32,6 +32,7 @@
 CONFIG_TFTP_TSIZE=y
 CONFIG_AT91_GPIO=y
 CONFIG_CMD_PCA953X=y
+CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_SOFT=y
 CONFIG_SYS_I2C_SOFT_SLAVE=0x7F
 # CONFIG_MMC is not set