sunxi: Properly check for SATAPWR and MACPWR

The #ifdef CONFIG_xxxPWR conditionals were not working as expected, as
string Kconfig symbols are always "defined" from the preprocessor's
perspective. This lead to unnecessary calls to the GPIO routines, but
also always added a half a second delay to wait for a SATA disk to power
up. Many thanks to Peter for pointing this out!

Fix this by properly comparing the Kconfig symbols against the empty
string. strcmp() would be nicer for this, but GCC does not optimise this
away, probably due to our standalone compiler switches.

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Samuel Holland <samuel@sholland.org> # Orange Pi WinPlus
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>