power: reset: at91-reset: change the power on reason prototype

It is quite uncommon to use a driver helper with parameters like *pdev
and __iomem *base. It is much cleaner and close to today's standards to
provide the per-device driver structure and then access its
internals. Let's do this with the helper which returns the power on
reason. While we change the parameters, we can as well rename the
function from at91_reset_status() to at91_reset_reason() to be more
accurate with what the helper actually does, and finally because we don't
really need the pdev argument in this helper besides for printing the
reset reason, we can move the dev_info() call into the probe.

All these modifications prepare the introduction of a sysfs entry to
access this information. This way the diff will be much smaller. Thus,
there is no intended functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>