[drivers] pinmux

[arunthomas]

Description

Implement Tock driver for pinmux

Tasks

• [ ] Audit the driver for missing functionality
• [ ] Implement any missing driver functionality
• [ ] Implement missing HIL functionality (if needed)
• [ ] Implement tests

[rnongie]

Note - This task has been simplified to focus on implementing driver functionality. The "implement driver tests" has been moved to #33

Please feel free to close this task when driver functionality is complete.