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/configs/am335x_shc_prompt_defconfig b/configs/am335x_shc_prompt_defconfig
index 27abd5e..41b9d8b 100644
--- a/configs/am335x_shc_prompt_defconfig
+++ b/configs/am335x_shc_prompt_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SERIES=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_FIT=y