mips: rs870e: stop exporting local functions

These four functions are exported, but don't have any users, and no
prototypes, which now causes warnings:

drivers/platform/mips/rs780e-acpi.c:35:6: error: no previous prototype for 'pm_iowrite' [-Werror=missing-prototypes]
drivers/platform/mips/rs780e-acpi.c:41:4: error: no previous prototype for 'pm_ioread' [-Werror=missing-prototypes]
drivers/platform/mips/rs780e-acpi.c:47:6: error: no previous prototype for 'pm2_iowrite' [-Werror=missing-prototypes]
drivers/platform/mips/rs780e-acpi.c:53:4: error: no previous prototype for 'pm2_ioread' [-Werror=missing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-5-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>