macintosh/via-pmu: Avoid compiler warnings when CONFIG_PROC_FS is disabled

drivers/macintosh/via-pmu.c:897:12: warning: 'pmu_battery_proc_show' defined but not used [-Wunused-function]
 static int pmu_battery_proc_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~~~~~
drivers/macintosh/via-pmu.c:871:12: warning: 'pmu_irqstats_proc_show' defined but not used [-Wunused-function]
 static int pmu_irqstats_proc_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~~~~~~
drivers/macintosh/via-pmu.c:860:12: warning: 'pmu_info_proc_show' defined but not used [-Wunused-function]
 static int pmu_info_proc_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~~

Add some #ifdefs to avoid unused code warnings when CONFIG_PROC_FS is
disabled.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0c11c0770fc4ec7e80a4b2e0ffce1055b792cfdb.1647854880.git.fthain@linux-m68k.org