The DC power STS shouldn't be checked if booting off 5V supply.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Robert Deliën <robert@delien.nl>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>