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/Bananapi_M2_Ultra_defconfig b/configs/Bananapi_M2_Ultra_defconfig
index 5db7e90..8896d2a 100644
--- a/configs/Bananapi_M2_Ultra_defconfig
+++ b/configs/Bananapi_M2_Ultra_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_I2C=y
 CONFIG_SCSI_AHCI=y
+CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_MVTWSI=y
 CONFIG_RGMII=y
 CONFIG_SUN8I_EMAC=y