iio: Allow to read mount matrix from ACPI

Currently mount matrix is allowed in Device Tree, though there is
no technical issue to extend it to support ACPI.

Convert the function to use device_property_read_string_array() and
thus allow to read mount matrix from ACPI if available.

Example of use in _DSD method:

  Name (_DSD, Package ()
  {
     ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
     Package ()
     {
        Package () { "mount-matrix", Package() {
                "1", "0",     "0",
                "0", "0.866", "0.5",
                "0", "-0.5",  "0.866",
        } },
     }
  })

At the same time drop the "of" prefix from its name and
convert current users.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>