powerpc/fadump: update documentation about option to release opalcore

With /sys/firmware/opal/core support available on OPAL based machines
and an option to the release memory used by kernel in exporting this
core file, update FADump documentation with these details.
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821382786.5656.13173494907671241231.stgit@hbathini.in.ibm.com

Documentation/powerpc/firmware-assisted-dump.rst

@@ -110,6 +110,16 @@ capture kernel boot to process this crash data. Kernel config
 option CONFIG_PRESERVE_FA_DUMP has to be enabled on such kernel
 to ensure that crash data is preserved to process later.
 
+-- On OPAL based machines (PowerNV), if the kernel is build with
+   CONFIG_OPAL_CORE=y, OPAL memory at the time of crash is also
+   exported as /sys/firmware/opal/core file. This procfs file is
+   helpful in debugging OPAL crashes with GDB. The kernel memory
+   used for exporting this procfs file can be released by echo'ing
+   '1' to /sys/kernel/fadump_release_opalcore node.
+
+   e.g.
+     # echo 1 > /sys/kernel/fadump_release_opalcore
+
 Implementation details:
 -----------------------
 
@@ -273,6 +283,15 @@ Here is the list of files under kernel sysfs:
     enhanced to use this interface to release the memory reserved for
     dump and continue without 2nd reboot.
 
+ /sys/kernel/fadump_release_opalcore
+
+    This file is available only on OPAL based machines when FADump is
+    active during capture kernel. This is used to release the memory
+    used by the kernel to export /sys/firmware/opal/core file. To
+    release this memory, echo '1' to it:
+
+    echo 1  > /sys/kernel/fadump_release_opalcore
+
 Here is the list of files under powerpc debugfs:
 (Assuming debugfs is mounted on /sys/kernel/debug directory.)