gpio/stm32: Move from the OPEN_DRAIN interface to the DRIVE_STRENGTH interface

The STM32 GPIO driver extended the generic GPIO driver interace with
the concept of OPEN_DRAIN.  There is previous discussion about
representing such concepts in the GPIO interface in a more general
fashion here:

https://lists.zephyrproject.org/archives/list/devel@lists.zephyrproject.org/thread/6DCFUAKCOOOBHUO3ZK45ES6IQXOEOFWN/



The DRIVE STRENGTH interface supports the concepts of OPEN DRAIN and
other variants supports by other vendors hardware.

Adjust the STM32 GPIO driver to use the DRIVE STRENGTH interface.  A
following patch will address the simplication of gpio.h

Change-Id: I56b0792ec2b21f1adc673dff019288dc8573d005
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>