dts: arm: atmel: samx7x: refactor devicetree files for the Atmel SAMx7x

Refactor the devicetree 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.

Introduce a base samx7x.dtsi for all members of the family, containing the
union of all supported peripherals. Specific product series can use
/delete-node/ in their DTSI (e.g. same70.dtsi) for removing peripherals not
present in that product series.

Replace pin-count-specific DTSI files (e.g. same70q19b.dtsi) with
pin-count-agnostic DTSI files (e.g. same70x19b.dtsi) as the pin-count is
not taken into account in these anyways, and adjust the relevant board
devicetrees accordingly.

As part of this refactoring, introduce support for the missing flash memory
density variants of the SAM E70 product series.

Support for the two remaining product series (SAM S70/V70) is not part of
this refactoring as these will require further changes to the SoC support
code (soc/atmel/sam/).

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