rockchip: clk: rk3399: add clocking support for Ethernet

rockchip: clk: rk3399: add clocking support for Ethernet

The Ethernet driver for the RK3288/3399 GMAC makes sure that the clock
is ungated through a call to clk_set_rate(...). Even though nothing
needs to be done on the RK3399 (the clock gates are open and the clock
is external), we need to implement enough support to at least return
success to enable driver probing.

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>