clk: meson: meson8b: change references to the XTAL clock to use [fw_]name

The XTAL clock is an actual crystal which is mounted on the PCB. Thus
the meson8b clock controller driver should not provide the XTAL clock.

The meson8b clock controller driver must not use references to
the meson8b_xtal clock anymore before we can provide the XTAL clock
via OF. Replace the references to the meson8b_xtal.hw by using
clk_parent_data's .fw_name and .name = "xtal" (along with index = -1).
This makes the common clock framework use the clock provided via OF and
if that's not available it falls back to getting the clock by it's name
(which is then the clk_fixed_rate which we register in our driver).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>