net: stmmac: dwmac-meson8b: simplify clock registration

To goal of this patch is to simplify the registration of the RGMII TX
clock (and it's parent clocks). This is achieved by:
- introducing the meson8b_dwmac_register_clk helper-function to remove
  code duplication when registering a single clock (this saves a few
  lines since we have 4 clocks internally)
- using devm_add_action_or_reset to disable the RGMII TX clock
  automatically when needed. This also allows us to re-use the standard
  stmmac_pltfr_remove function.
- devm_kasprintf() and devm_kstrdup() are not used anymore to generate
  the clock name (these are replaced by a variable on the stack) because
  the common clock framework already uses kstrdup() internally.

No functional changes intended.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>