ACPICA: Rename ACPI bit register access functions

Rename acpi_get_register and acpi_set_register to clarify the
purpose of these functions. New names are acpi_read_bit_register
and acpi_write_bit_register.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>