clk: rockchip: rk3568: Add dummy support for GMAC speed clocks

Pine64 Quartz64 boards DT reference SCLK_GMAC1_RGMII_SPEED in the
assigned-clocks property of the gmac1 node. This result in a ENOENT
error when driver core tries to set a parent for this clock.

The clock speed in rgmii/rmii mode is changed using clk_set_rate of the
tx_rx clock and not using clk_set_parent of the speed clock.

Add dummy support for SCLK_GMAC1_RGMII_SPEED and similar clocks to clk
driver to allow a driver for gmac node to probe.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>