powerpc: Document xmon options

Document all options currently supported by xmon debugger.
Signed-off-by: Gustavo Romero <gromero@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190814205638.25322-1-gromero@linux.ibm.com

Documentation/admin-guide/kernel-parameters.txt

@@ -5305,3 +5305,22 @@
 			A hex value specifying bitmask with supplemental xhci
 			host controller quirks. Meaning of each bit can be
 			consulted in header drivers/usb/host/xhci.h.
+
+	xmon		[PPC]
+			Format: { early | on | rw | ro | off }
+			Controls if xmon debugger is enabled. Default is off.
+			Passing only "xmon" is equivalent to "xmon=early".
+			early	Call xmon as early as possible on boot; xmon
+				debugger is called from setup_arch().
+			on	xmon debugger hooks will be installed so xmon
+				is only called on a kernel crash. Default mode,
+				i.e. either "ro" or "rw" mode, is controlled
+				with CONFIG_XMON_DEFAULT_RO_MODE.
+			rw	xmon debugger hooks will be installed so xmon
+				is called only on a kernel crash, mode is write,
+				meaning SPR registers, memory and, other data
+				can be written using xmon commands.
+			ro 	same as "rw" option above but SPR registers,
+				memory, and other data can't be written using
+				xmon commands.
+			off	xmon is disabled.