devicetree.h: Add macro DT_INST_FOREACH

With this macro device drivers can call macros and functions
on every device instance compatible to that driver.
This makes it possible to make drivers agnostic to the
potential counts of instances.
Sidenote: Introduces helper macro DT_CALL_WITH_ARG.

Signed-off-by: Ioannis Papamanoglou <iopapamanoglou@gmail.com>