board: samsung: e850-96: Extract device info from fw loading code

Make it possible to provide the information about storage device where
LDFW firmware resides to the firmware loading routine. The firmware
loader code shouldn't have that data hard-coded anyway, and it also
allows for implementing more dynamic behavior later, like choosing the
storage device containing LDFW via some environment variables.

No functional change.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>