Revert "genirq: Restrict export of irq_to_desc()"

This reverts commit 64a1b95b and
commit 11cc92eb.

Commit 64a1b95b ("genirq: Restrict export of irq_to_desc()") removed
the export of irq_to_desc() and commit 11cc92eb ("genirq: Fix export
of irq_to_desc() for powerpc KVM") added it back with the powerpc KVM
limitation. But there's still some usage in GKI kernel modules, so
revert both two commits.

Bug: 199823932
Signed-off-by: Huang Yiwei <hyiwei@codeaurora.org>
Change-Id: I3f871ecb5bc002c8643b4ddd3137a6899ac77b4f