sys: util: Make FIELD_* available from device tree

Move FIELD_* macros from zephyr/sys/util.h to zephyr/sys/util_macro.h,
as zephyr/sys/util.h cannot be used from device tree.

Device tree uses zephyr/dt-bindings/dt-util.h, which includes
zephyr/sys/util_macro.h.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>