ipmi watchdog : add panic_wdt_timeout parameter

ipmi watchdog : add panic_wdt_timeout parameter

In order to allow panic actions to be processed, the ipmi watchdog
driver sets a new timeout value on panic. The 255s timeout
was designed to allow kdump and others actions on panic, as in
http://lkml.iu.edu/hypermail/linux/kernel/0711.3/0258.html

This is counter-intuitive for a end-user who sets watchdog timeout
value to something like 30s and who expects BMC to reset the system
within 30s of a panic.

This commit allows user to configure the timeout on panic.

Signed-off-by: Jean-Yves Faye <jean-yves.faye@c-s.fr>