rpmsg: fix dependency on initialization order

When rpmsg drivers are built into the kernel, they must not initialize
before the rpmsg bus does, otherwise they'd trigger a BUG() in
drivers/base/driver.c line 169 (driver_register()).

To fix that, and to stop depending on arbitrary linkage ordering of
those built-in rpmsg drivers, we make the rpmsg bus initialize at
subsys_initcall.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Federico Fuga <fuga@studiofuga.com>
[ohad: rewrite the commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>