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

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/781cafb0-8d06-8b56-907a-5175c2da196a@gmail.com

Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Doug Berger <opendmb@gmail.com>
Cc: Justin Chen <justinpopo6@gmail.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Markus Mayer <mmayer@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Link: https://lore.kernel.org/r/20220719195325.402745-5-gpiccoli@igalia.com
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>