boot/fit: declare (and use) new constant for conf's compatible prop

Fit conf node may have a compatible property[1] which stores the root
compatible of the first blob in the fdt property of the node. This can
be used to automatically select the proper conf node based on the
compatible from the running U-Boot (matching the former's compatible
with the latter)[2].

This adds (and uses) this constant for FIT node parsing.

Note that this property may also appear in fpga image nodes[3] but that
isn't done in this commit.

[1] https://fitspec.osfw.foundation/#optional-properties compatible paragraph
[2] https://fitspec.osfw.foundation/#select-a-configuration-to-boot
[3] https://fitspec.osfw.foundation/#images-node 2.3.2 Conditionally mandatory property

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>

boot/image-fit.c

@@ -1756,7 +1756,7 @@ int fit_conf_find_compat(const void *fit, const void *fdt)
 			continue;
 
 		/* If there's a compat property in the config node, use that. */
-		if (fdt_getprop(fit, noffset, "compatible", NULL)) {
+		if (fdt_getprop(fit, noffset, FIT_COMPAT_PROP, NULL)) {
 			fdt = fit;		  /* search in FIT image */
 			compat_noffset = noffset; /* search under config node */
 		} else {	/* Otherwise extract it from the kernel FDT. */