staging: board: Initialize staging board code earlier

Currently the staging board code is initialized from a late_initcall().
However, unused PM domains are also disabled from a late_initcall(),
which happens before due to link order.

Change the initialization of staging board code from using
late_initcall() to device_initcall() to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>