watchdog: add CONFIG_WATCHDOG_NOWAYOUT support to PowerPC Book-E watchdog driver

Normally, the watchdog is disabled when dev/watchdog is closed, but if
CONFIG_WATCHDOG_NOWAYOUT is defined, then it means that the watchdog should
remain enabled.  So we should disable it only if CONFIG_WATCHDOG_NOWAYOUT is
not defined.

Also ensure that /dev/watchdog is only opened by one process at a time.  That
way, a second process can't accidentally disable the watchdog while the first
process has it open.  There shouldn't be any need for more than one process to
open /dev/watchdog anyway.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>