Convert CONFIG_SPL_POWER_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 667e15c..5f22e8c 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -33,3 +33,6 @@
 
 config SPL_NAND_SUPPORT
 	default y
+
+config SPL_POWER_SUPPORT
+	default y