ADC legacy

* doc: mark legacy adc API as deprecated
 * this is just to prevent confusion when browsing the reference
* drivers: mark adc.h as legacy
 * rename file to reflect the status, and prevent confusion when browsing the files
* cpu: use adc_legacy.h where appropriate
 * the deprecated header had not even been used
* cpu/mc1322x: rename adc header to mc1322x-adc
 * would have resolved a name conflict, offers unification (with lpc2378) and prevents confusion now