io: Allow image load to address zero

Remove assert on buffer address equal zero.
Marvell uses address 0x0 for loading BL33,
so this check is irrelevant and breaks the
debug builds on Marvell platforms.

Change-Id: Ie56a51138e2e4ddd8986dd7036797dc2d8b10125
Signed-off-by: Haim Boot <hayim@marvell.com>
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-on: http://vgitil04.il.marvell.com:8080/54589