To simplify board definitions and for unification between samd2x and
newer models, don't use the GCLK bitmask in board definitions.
Instead use the GCLK index and generate the bitmask when needed.
This change is for all boards with a sam0 cpu. This cpu just has one ADC.
It is unnecessary to have defines with ADC_0_ prefix as if multiple ADCs
are possible.
Some defines were not used, such as ADC_0_EN, ADC_0_CHANNELS,
ADC_MAX_CHANNELS, ADC_0_CLK_SOURCE, ADC_0_CHANNELS
Change all ADC_0_ prefixes to ADC_
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
The currently supported SAM0 MCUs (samd21, saml21, saml1x) share the same
Timer peripheral, yet each of them carries it's own copy of the Timer
driver.
This introduces a new timer driver that is common for all sam0 MCUs and
uses structs for configuration instead of defines.
