dts: atmel: sam0: Explicity disable nodes

When running tests on sam0 platform was detected that pinctrl for ADC
were not defined for some boards. To force an error at build time
nodes should be explicity disabled. This explicity disable nodes on
devicetree that require some user configuration.

In addition, the adc feature were excluded in some boards and
samr21-xpro was correct updated.

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