Add driver API reentrancy support to PWM shim driver

[zephyrbot]

Reported by Kuo-Lang Tseng:

Global data and device IO registers being set by the driver API can be messed up in the middle of the API call when a concurrent call (from other fiber/task) to the same API is made.

(Imported from Jira ZEP-430)

[zephyrbot]

by Kuo-Lang Tseng:

https://gerrit.zephyrproject.org/r/#/c/3414/

[zephyrbot]

by ethan gao:

Verified to operate PWM concurrently within multi-fibers