i2c: Convert CONFIG_POWER_I2C et al to Kconfig

This converts the following to Kconfig:
   CONFIG_POWER_I2C
   CONFIG_POWER_LEGACY

They are handled at the same time due to a dependency between them.
Update the Makefile rule to use legacy power only in U-Boot proper.
Unfortunately a separate rule is needed in SPL to be able to build
legacy power.  Add SPL related symbols for both, to allow for SPL-only
usage.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
[trini: More SPL related cleanups, reword commit message]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/power/Makefile b/drivers/power/Makefile
index ab52bc1..801b727 100644
--- a/drivers/power/Makefile
+++ b/drivers/power/Makefile
@@ -16,9 +16,9 @@
 obj-$(CONFIG_TWL4030_POWER)	+= twl4030.o
 obj-$(CONFIG_TWL6030_POWER)	+= twl6030.o
 obj-$(CONFIG_PALMAS_POWER)	+= palmas.o
-obj-$(CONFIG_POWER_LEGACY) += power_core.o
+obj-$(CONFIG_$(SPL_TPL_)POWER_LEGACY) += power_core.o
 obj-$(CONFIG_DIALOG_POWER) += power_dialog.o
 obj-$(CONFIG_POWER_FSL) += power_fsl.o
-obj-$(CONFIG_POWER_I2C) += power_i2c.o
+obj-$(CONFIG_$(SPL_TPL_)POWER_I2C) += power_i2c.o
 obj-$(CONFIG_POWER_SPI) += power_spi.o
 obj-$(CONFIG_POWER_MT6323) += mt6323.o