device: perform dynamic device initialization during system startup

Initialize all device objects in a batch before invoking any code that
might try to reference data in them.  This eliminates a race condition
enabled by the ability to resolve a device structure at build time,
and reference it from one device's initialization routine before the
device itself has been initialized.

While the device is pulled from the sys_init records rather than
static devices, all in-tree init_entry records that are associated
with devices are produced via Z_DEVICE_DEFINE(), so there should be no
static devices that would be missed by instead iterating over the
device records.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>