staging: greybus: power_supply: remove KERNEL_VERSION checks

No need to support older kernel versions in the Greybus Power Supply
driver, so remove the checks as needed, we can now rely on all of the
correct Power Supply core apis being present. Also move some properties
definitions to the power supply greybus code.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>