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/include/i2c.h b/include/i2c.h
index 3d9ecab..0786705 100644
--- a/include/i2c.h
+++ b/include/i2c.h
@@ -762,7 +762,7 @@
 void i2c_init(int speed, int slaveaddr);
 void i2c_init_board(void);
 
-#ifdef CONFIG_SYS_I2C_LEGACY
+#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 /*
  * i2c_get_bus_num:
  *
@@ -942,7 +942,7 @@
  * only for backwardcompatibility, should go away if we switched
  * completely to new multibus support.
  */
-#if defined(CONFIG_SYS_I2C_LEGACY) || defined(CONFIG_I2C_MULTI_BUS)
+#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY) || defined(CONFIG_I2C_MULTI_BUS)
 # if !defined(CONFIG_SYS_MAX_I2C_BUS)
 #  define CONFIG_SYS_MAX_I2C_BUS		2
 # endif