watchdog: f71808e_wdt: rename variant-independent identifiers appropriately

Code for the common parts of the driver either uses watchdog_ as
prefix for the watchdog API or f71808e_ for everything else.

The driver now supports 9 more variants besides the f71808e,
so let's rename the common parts to start with fintek_wdt_ instead.

This makes code browsing easier, because it's readily apparent
that functions are not variant-specific. Some watchdog_-prefixed
functions remain, but these will be dropped altogether with the move
to the kernel watchdog API in a later commit.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/31805c6aeb8d161f852ddad7c32b91319f924988.1628525954.git-series.a.fatoum@pengutronix.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>