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/board/Arcturus/ucp1020/spl.c b/board/Arcturus/ucp1020/spl.c
index f7c4960..4a70a21 100644
--- a/board/Arcturus/ucp1020/spl.c
+++ b/board/Arcturus/ucp1020/spl.c
@@ -106,7 +106,7 @@
 	env_relocate();
 #endif
 
-#ifdef CONFIG_SYS_I2C_LEGACY
+#if CONFIG_IS_ENABLED(SYS_I2C_LEGACY)
 	i2c_init_all();
 #else
 	i2c_init(CONFIG_SYS_I2C_SPEED, CONFIG_SYS_I2C_SLAVE);