powerpc: use read_file() in kernel and initrd loading

Replace the unsafe read-loops in the powerpc kernel image loading
function with our new and safe read_file() wrapper.
This should fix random fails in kernel image loading, especially
from pipes and sockets.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>