[WATCHDOG] rc32434_wdt: add timeout module parameter

The WDT timer ticks quite fast (half of the CPU clock speed, which may
be between 198MHz and 330MHz (or 400MHz on newer boards)). Given it's
size of 32Bit, the maximum timeout value ranges from about 21s to 43s,
depending on the configured CPU clock speed.

This patch add's the timeout module parameter and checks that it's not
bigger then the maximum timeout for the given clock speed.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>