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/k2l_evm_defconfig b/configs/k2l_evm_defconfig
index 62b7877..91d5bd4 100644
--- a/configs/k2l_evm_defconfig
+++ b/configs/k2l_evm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_K2L_EVM=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="k2l-evm"
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_VERSION_VARIABLE=y