[WATCHDOG] w83697hf/hg WDT driver - patch 7

This is patch 7 in the series of patches that converts
Marcus Junker's w83697hf watchdog driver to Samuel Tardieau's
w83697hf/hg watchdog driver.

This patch contains following changes:
 - add w83697hf_unlock function to enter the
   chipsets extended function mode.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>