watchdog: omap_wdt: Use devm_ioremap_resource()

Use devm_ioremap_resource() in order to make the code simpler,
and remove 'struct resource *mem' from 'struct omap_wdt_dev'
and omap_wdt_probe(), resplectively. because the 'mem' variables
are not used anymore. Also the redundant return value check of
platform_get_resource() is removed, because the value is checked
by devm_ioremap_resource().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>