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/date.c b/cmd/date.c
index 149ca42..0e2dfbc 100644
--- a/cmd/date.c
+++ b/cmd/date.c
@@ -49,7 +49,7 @@
 			return CMD_RET_FAILURE;
 		}
 	}
-#elif defined(CONFIG_SYS_I2C_LEGACY)
+#elif CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 	old_bus = i2c_get_bus_num();
 	i2c_set_bus_num(CONFIG_SYS_RTC_BUS_NUM);
 #else
@@ -122,7 +122,7 @@
 	}
 
 	/* switch back to original I2C bus */
-#ifdef CONFIG_SYS_I2C_LEGACY
+#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 	i2c_set_bus_num(old_bus);
 #elif !defined(CONFIG_DM_RTC)
 	I2C_SET_BUS(old_bus);