qmsi: wdt: improved reentrancy of the wdt driver

Uses locking in an API and removes a global variable
for improved safety/correctness in the face of concurrent
threads and fibers.

The locking mechanism is by default disabled in the
build. To enable it, the following flag needs to be defined:

CONFIG_WDT_QMSI_API_REENTRANCY

Jira: ZEP-440
Change-Id: Ibcd1501a4af628017b20d7e7ce20b988c8e4e4e2
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>