clk: rk3399: Add enable/disable clks

Yes, most of the high speed peripheral clocks
in rk3399 enabled by default.

But it would be better to handle them via clk
enable/disable API for handling proper reset
conditions like 'usb reset' over command line.

So, enable USB, GMAC clock via enable/disable ops.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Suniel Mahesh <sunil.m@amarulasolutions.com> # roc-rk3399-pc
Tested-by: Suniel Mahesh <sunil@amarulasolutions.com> #roc-rk3399-pc
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>