[WATCHDOG] rc32434_wdt.c: use resource_size()

The size value passed to ioremap_nocache() is not correct.
Use resource_size() to get the correct value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>