ARM: at91/dt: split sama5d3 peripheral definitions

This patch splits the sama5d3 SoCs definition:
- a common base for all sama5d3 SoCs (sama5d3.dtsi)
- several optional peripheral definitions which will be included by sama5d3
  specific SoCs (sama5d3_'periph name'.dtsi)
- sama5d3 specific SoC definitions (sama5d3x.dtsi)

This provides a better representation of the real hardware (drop unneed
dt nodes) and avoids peripheral id conflict (which is not the case for
current sama5d3 SoCs, but could be if other SoCs of this family are
released).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
[nicolas.ferre@atmel.com: add more "sama5d3?" compatibility strings]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>