soc: arm: atmel_sam: Add helper macros to init array of pins

Added ATMEL_SAM_DT_NUM_PINS macro which will return the number of pins
to initialize for "pinctrl-0" devicetree property.

Added ATMEL_SAM_DT_PINS macro which will create an array initializer
with all the pins associated with "pinctrl-0" devicetree property.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>