watchdog: bcm2835_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



- Drop assignments to otherwise unused variables
- Replace of_iomap() with platform_get_resource() followed by
  devm_ioremap_resource()
- Replace &pdev->dev with dev if 'struct device *dev' is a declared
  variable
- Use devm_watchdog_register_driver() to register watchdog device
- Replace shutdown function with call to watchdog_stop_on_reboot()

Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Lee Jones <lee@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>