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/wandboard_defconfig b/configs/wandboard_defconfig
index d3f0e0e..f5c9948 100644
--- a/configs/wandboard_defconfig
+++ b/configs/wandboard_defconfig
@@ -57,6 +57,7 @@
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_DWC_AHSATA=y
 CONFIG_DM_I2C=y
+CONFIG_SPL_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MXC=y
 CONFIG_FSL_USDHC=y
 CONFIG_PHYLIB=y