clk: meson: axg: claim clock controller input clock from DT

Instead of relying on a fixed name for the xtal clock, claim the
controller input clock trough DT.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20190116175435.4990-3-jbrunet@baylibre.com