power: Rearrange code to guard power command with CONFIG_SPL_BUILD guard

In order to discard this code when unused in SPL we need to guard the
command with a check for CONFIG_SPL_BUILD and we rearrange the code
slightly to make this cleaner.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@konsulko.com>