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/cmd/eeprom.c b/cmd/eeprom.c
index 7e1a514..ea389c3 100644
--- a/cmd/eeprom.c
+++ b/cmd/eeprom.c
@@ -52,7 +52,7 @@
 	/* I2C EEPROM */
 #if CONFIG_IS_ENABLED(DM_I2C)
 	eeprom_i2c_bus = bus;
-#elif defined(CONFIG_SYS_I2C_LEGACY)
+#elif CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 	if (bus >= 0)
 		i2c_set_bus_num(bus);
 	i2c_init(CONFIG_SYS_I2C_SPEED, CONFIG_SYS_I2C_SLAVE);