[fdt] Allow for parsing device trees where the length is known in advance

Allow for parsing device trees where an external factor (such as a
downloaded image length) determines the maximum length, which must be
validated against the length within the device tree header.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/prefix/sbiprefix.S

@@ -123,6 +123,7 @@ _sbi_start:
 	/* Register device tree */
 	la	a0, sysfdt
 	mv	a1, s1
+	li	a2, -1
 	call	fdt_parse
 
 	/* Call main program */