Add TI Tiva C controller support.

Currently supported:
 - GPIO
 - System clock configuration
 - UART TX with polling
 - Timer support with polling
 - Platform tree support
Rakefile: remove useless error message
Platform tree: rename gpio ports from 'PortX' into just 'x'
PT parser: change error message when parsing unexpected identifier
Add missing license headers
Move get_reg_ref to util/support.rs
Homogenize comments and TODOs
Fix pin.rs regio alignment
Rename sysctl::Periph into sysctl::PeripheralClock
Handle fatal errors by calling panic!