rockchip: clk: rk3288: handle clk_enable requests for GMAC

Since b0ba1e7e
(rockchip: clk: rk3288: add clk_enable function and support USB HOST0/HSIC)
Ethernet no longer probes on RK3288.

Add no-ops for GMAC clocks observed to be requested which match the
clk_enable cases in RK3368 and RK3399.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Cc: Wadim Egorov <w.egorov@phytec.de>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>