MIPS: Loongson64: Make acpi_registers_setup() static

sparse report build warning as follows:

drivers/platform/mips/rs780e-acpi.c:72:6: warning:
 symbol 'acpi_registers_setup' was not declared. Should it be static?

And function acpi_registers_setup() is not used outside of this file,
so marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>