power: test_power: Use enum as index for array of supplies

Replace hard-coded numbers for indices of power supply array with enum.
This improves a little the readability as one does not have to guess
which power supply is associated with number.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>