i2c: Convert CONFIG_POWER_I2C et al to Kconfig

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>