doc: "os" is also mandatory for "ramdisk" FIT image components

According to fit_image_print(), the "os" property from "image" node is required
also when "type=ramdisk".

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

doc/uImage.FIT/source_file_format.txt

@@ -166,10 +166,10 @@ the '/images' node should have the following layout:
     should be set to "none".
 
   Conditionally mandatory property:
-  - os : OS name, mandatory for type="kernel", valid OS names are: "openbsd",
-    "netbsd", "freebsd", "4_4bsd", "linux", "svr4", "esix", "solaris", "irix",
-    "sco", "dell", "ncr", "lynxos", "vxworks", "psos", "qnx", "u_boot",
-    "rtems", "unity", "integrity".
+  - os : OS name, mandatory for types "kernel" and "ramdisk". Valid OS names
+    are: "openbsd", "netbsd", "freebsd", "4_4bsd", "linux", "svr4", "esix",
+    "solaris", "irix", "sco", "dell", "ncr", "lynxos", "vxworks", "psos", "qnx",
+    "u_boot", "rtems", "unity", "integrity".
   - arch : Architecture name, mandatory for types: "standalone", "kernel",
     "firmware", "ramdisk" and "fdt". Valid architecture names are: "alpha",
     "arm", "i386", "ia64", "mips", "mips64", "ppc", "s390", "sh", "sparc",