dts: atmel: samr21: Use samd21 as base

The samr21 is a samd21 with a builtin at86rf233 radio. Use the samd21 as
base for these SoC and drop all duplicated nodes.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
(cherry picked from commit 01fc0a75)