mfd: pm8xxx: Move pm8xxx-irq.c contents into only driver that uses it

The pm8xxx-irq.c code is practically mandatory given that the
pm8921-core driver will WARN about it missing and the Kconfig
marks it as default y when a PM8xxx chips is enabled. The only
reason the file was split out was because we planned to support
other pm8xxx chips with different pm8xxx-core.c files. Now that
we have DT on ARM this isn't necessary because we should be able
to support all the ssbi based PM8xxx chips in one driver and one
file with no data bloat. Let's move this code into the only
driver that uses it right now (pm8921) so that it's always compiled when
needed. In the future we can rename pm8921-core.c to something
more generic.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>