rockchip: clk: rk3368: handle clk_enable requests for GMAC

rockchip: clk: rk3368: handle clk_enable requests for GMAC

Since commit ba1f96672522 ("net: designware: add clock support"), the
designware GMAC driver enables all referenced clocks. While this is a
no-op for the RK3368 during boot-up (reset behaviour has all the clock
gates open anyway), we still need to handle the clock-ids passed in
the enable op of the clock-driver and return a success.

This change extends the RK3368 clk driver to:
(a) provide a enable op
(b) signals success to the caller when the clocks for the GMAC are
    enabled (no actual action is necessary as the gates are open
    after reset)

References: commit ba1f96672522 ("net: designware: add clock support")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>