device_tree: qemu_fdt_getprop_cell converted to use the error API

device_tree: qemu_fdt_getprop_cell converted to use the error API

This patch aligns the prototype with qemu_fdt_getprop. The caller
can choose whether the function self-asserts on error (passing
&error_fatal as Error ** argument, corresponding to the legacy behavior),
or behaves differently such as simply output a message.

In this later case the caller can use the new lenp parameter to interpret
the error if any.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>