image-fit: Accept IH_TYPE_FIRMWARE in fit_image_load() as valid

Consider the following FIT:

	images {
		whipple {};
	};
	configurations {
		conf-1 {
			firmware = "whipple";
		};
	};

Getting the 'firmware' image with fit_image_load() is not possible, as
it doesn't understand 'firmware =' properties. Although one could pass
IH_TYPE_FIRMWARE for 'image_type', this needs to be converted to a
"firmware" string for FDT lookup -- exactly what this change does.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>