[WATCHDOG] SA1100 watchdog maximum timeout

This patch replaces the hardcoded 255 seconds limit for a real limit based on
oscr_freq.

Also, the 'firmware_version' field is changed to '1' to allow the user
space application to easily detect that this driver supports a higher
maximum timeout.

Signed-off-by: Raphael Assenat <raph@8D.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>