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_igep0033_defconfig b/configs/am335x_igep0033_defconfig
index 3dbde19..f859d38 100644
--- a/configs/am335x_igep0033_defconfig
+++ b/configs/am335x_igep0033_defconfig
@@ -8,12 +8,13 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MTD_SUPPORT=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set