Add support for Timers (TC)

[haata]

atsam4s_a, atsam4s_b

• 1 Timer Module (3 Channels)
• 16bit registers atsam4s_c
• 2 Timer Modules (2x3 Channels)
• 16bit registers atsam4e
• 3 Timer Modules (3x3 Channels)
• 32bit registers

Only basic timer functionality for now. Out-of-scope features

• Wave mode
• Chaining timer channels
• External clock input

[haata]

Still trying the figure out the best way to do this. Currently the problem is that there are can be multiple Timer Modules (usually 0 through 2). And each module has 3 channels.

For the most part each channel is an independent channel and can be configured separately. Unfortunately there are some shared registers in the module as well. On top of that, I haven't figured out a way to assign specific registers to a struct yet. Ideally, each Timer Module Channel would own their specific registers. The Timer Module would own any of the generic registers for the module.

[haata]

Fixed with PR #47