soc: bcm: brcmstb: Document panic notifier action and remove useless header

The panic notifier of this driver is very simple code-wise, just a
memory write to a special position with some numeric code. But this
is not clear from the semantic point-of-view, and there is no public
documentation about that either.

After discussing this in the mailing-lists [0] and having Florian
explained it very well, document that in the code for the future
generations asking the same questions. Also, while at it, remove
a useless header.

[0] https://lore.kernel.org/lkml/[e-mail]

Cc: Brian Norris <[e-mail]>
Cc: Doug Berger <[e-mail]>
Cc: Justin Chen <[e-mail]>
Cc: Lee Jones <[e-mail]>
Cc: Markus Mayer <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Signed-off-by: Guilherme G. Piccoli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Florian Fainelli <[e-mail]>

drivers/soc/bcm/brcmstb/pm/pm-arm.c

···
 #include <linux/kernel.h>
 #include <linux/memblock.h>
 #include <linux/module.h>
-#include <linux/notifier.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/panic_notifier.h>
···
 
 	return of_io_request_and_map(dn, index, dn->full_name);
 }
-
+/*
+ * The AON is a small domain in the SoC that can retain its state across
+ * various system wide sleep states and specific reset conditions; the
+ * AON DATA RAM is a small RAM of a few words (< 1KB) which can store
+ * persistent information across such events.
+ *
+ * The purpose of the below panic notifier is to help with notifying
+ * the bootloader that a panic occurred and so that it should try its
+ * best to preserve the DRAM contents holding that buffer for recovery
+ * by the kernel as opposed to wiping out DRAM clean again.
+ *
+ * Reference: comment from Florian Fainelli, at
+ * https://lore.kernel.org/lkml/[e-mail]
+ */
 static int brcmstb_pm_panic_notify(struct notifier_block *nb,
 		unsigned long action, void *data)
 {