powerpc/via-pmu: Fix section mismatch warning

Make the struct via_pmu_driver const to avoid following warning:

WARNING: vmlinux.o(.data+0x4739c): Section mismatch in reference from the variable via_pmu_driver to the function .init.text:pmu_init()
The variable via_pmu_driver references
the function __init pmu_init()
If the reference is valid then annotate the
variable with __init* or __refdata (see linux/init.h) or name the variable:
*_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Suggested-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>