powerpc: use device_initcall for registering rtc devices

Currently these two RTC devices are in core platform code
where it is not possible for them to be modular.  It will
never be modular, so using module_init as an alias for
__initcall can be somewhat misleading.

Fix this up now, so that we can relocate module_init from
init.h into module.h in the future.  If we don't do this, we'd
have to add module.h to obviously non-modular code, and that
would be a worse thing.

Note that direct use of __initcall is discouraged, vs. one
of the priority categorized subgroups.  As __initcall gets
mapped onto device_initcall, our use of device_initcall
directly in this change means that the runtime impact is
zero -- they will remain at level 6 in initcall ordering.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Geoff Levand <geoff@infradead.org>
Acked-by: Geoff Levand <geoff@infradead.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>