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/README b/README
index a16afea..8c48b0b 100644
--- a/README
+++ b/README
@@ -1461,11 +1461,7 @@
 		In such cases CONFIG_GPIO_LED_INVERTED_TABLE may be defined
 		with a list of GPIO LEDs that have inverted polarity.
 
-- I2C Support:	CONFIG_SYS_I2C_LEGACY
-
-		Note: This is deprecated in favour of driver model. Use
-		CONFIG_DM_I2C instead.
-
+- I2C Support:
 		- drivers/i2c/fsl_i2c.c:
 		  - activate i2c driver with CONFIG_SYS_I2C_FSL
 		    define CONFIG_SYS_FSL_I2C_OFFSET for setting the register