dts: clock: Add atmel,assigned-clock property

Some platforms require special clock selection options. This could be
made using the already defined assigned-clocks from Linux clocks.

  See https://github.com/devicetree-org/dt-schema/blob/93ee8008959e362548168eaa7bdc20c115f3d586/dtschema/schemas/clock/clock.yaml#L24



This introduces the vendor atmel,assigned-clocks and
atmel,assigned-clock-names properties to generalize those conditions
in Zephyr for Atmel sam0 SoC series.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>