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.
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.
The following features were tested (briefly):
* UART (the first)
* I2C using a BMP180 on a SODAQ TPH board
* SPI0, connected to the on-board serial data flash (only DevID was read
using the periph_spi test program)
* xtimer is working (it's the same code as in samr21)
boards/sodaq-autonomo: Disable the "big" unittests because it does not fit