init: add sub-priority to internal ordering

Add a sub-priority field to `Z_INIT_ENTRY_SECTION`, which is used to
ensure that multiple drivers running at the same init priority still
run in an order that respects their devicetree dependencies.

This is primarily useful when multiple instances of the same device are
instantiated that can depend on each other, for example power domains
and i2c muxes.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>