Platform: Add configuration variables: ibm,shadow-boot-device ibm,last-booted

Issue:  (02/19/2020 02:00:52 PM) Document already-implemented NVRAM variables

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>

Platform/ch_nonvolatile_memory.xml

@@ -630,6 +630,12 @@ unsigned int nbytes; /* number of bytes to sum */
               <listitem>
                 <para><emphasis role="bold"><literal>reboot-command</literal></emphasis> [*]</para>
               </listitem>
+              <listitem>
+                <para><emphasis role="bold"><literal>ibm,shadow-boot-device</literal></emphasis> [1]</para>
+              </listitem>
+              <listitem>
+                <para><emphasis role="bold"><literal>ibm,last-booted</literal></emphasis> [1]</para>
+              </listitem>
             </itemizedlist>
           </section>