arm: at91: use macro to declare soc boot data

Instead of check the pointer of the init function, check the new builtin bool
to known if the soc is enabled.

This is needed as with the switch to the pinctrl the init will be NULL on pure
DT SoC.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>