watchdog: sprd_wdt: Convert to use device managed functions and other improvements

watchdog: sprd_wdt: Convert to use device managed functions and other improvements

Use device managed functions to simplify error handling, reduce
source code size, improve readability, and reduce the likelyhood of bugs.
Other improvements as listed below.

The conversion was done automatically with coccinelle using the
following semantic patches. The semantic patches and the scripts
used to generate this commit log are available at
https://github.com/groeck/coccinelle-patches

- Replace devm_add_action() followed by failure action with
  devm_add_action_or_reset()
- Replace 'val = e; return val;' with 'return e;'
- Introduce local variable 'struct device *dev' and use it instead of
  dereferencing it repeatedly
- Use devm_watchdog_register_driver() to register watchdog device

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