qmsi: pwm: Use locking mechanism to guard critical regions

Guard critical section of the driver API, so that multiple
simultaneous calls from tasks/fibers won't corrupt the driver
state.

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

CONFIG_PWM_QMSI_API_REENTRANCY

Jira: ZEP-430

Change-Id: Ia3eb2a962f4176a3ac94163b9843ab068abe4b4e
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>