UPSTREAM: clk: rockchip: add a dummy clock for the watchdog pclk on rk3399

UPSTREAM: clk: rockchip: add a dummy clock for the watchdog pclk on rk3399

Like rk3288, the pclk supplying the watchdog is controlled via the
SGRF register area. Additionally the SGRF isn't even writable in
every boot mode.

But still the clock control is available and in the future someone
might want to use it. Therefore define a simple clock for the time
being so that the watchdog driver can read its rate.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Reviewed-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
(cherry picked from commit git.kernel.org mmind/linux-rockchip.git
volatile-v4.8-clk/next e3d86c1a2295184374cf25cdb525e68a93b0ff90)

Change-Id: I616846d389d324be529966c63820e8707c7d428f
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>