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/drivers/i2c/Makefile b/drivers/i2c/Makefile
index c2eb24e..3a7ecd9 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -11,7 +11,7 @@
 obj-$(CONFIG_$(SPL_)I2C_CROS_EC_LDO) += cros_ec_ldo.o
 
 obj-$(CONFIG_I2C_MV) += mv_i2c.o
-obj-$(CONFIG_SYS_I2C_LEGACY) += i2c_core.o
+obj-$(CONFIG_$(SPL_)SYS_I2C_LEGACY) += i2c_core.o
 obj-$(CONFIG_SYS_I2C_ASPEED) += ast_i2c.o
 obj-$(CONFIG_SYS_I2C_AT91) += at91_i2c.o
 obj-$(CONFIG_SYS_I2C_CADENCE) += i2c-cdns.o