init: Support fine-grained device initialization priorities

Introduces the SYS_DEFINE_DEVICE() macro, which supports 5 distinct
levels of device initialization and 100 priorities within each level.

Note: The existing init macros (e.g. nano_early_init()) have been
adapted to utilize the enhanced initialization model, but will
eventually be retired.

Change-Id: If677029d8b711a3fae9b2f32b5470cd97d19aeda
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>