soc: atmel: samx7x: refactor SoC support for the Atmel SAM E70/V71

Refactor and merge the SoC support files for the Atmel SAM E70 and SAM V71
product series. These SoCs are part of a larger product family (SAM
E70/S70/V70/V71) and share a common set of peripherals.

Support for the two remaining product series (SAM S70/V70) is not part of
this refactoring as these will require further additions to the Atmel HAL.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>