device: macros for obtaining `struct init_entry`
device: macros for obtaining `struct init_entry`

Add macros for obtaining the `struct init_entry` associated with a
device, either from the devicetree node or by name.

Signed-off-by: Jordan Yates <d74a90e280bbad1cc3646781a4e234b467b60fe1@data61.csiro.au>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr