clk: rockchip: add watchdog pclk on rk3328

The watchdog pclk is controlled from the secure GRF but we still
want to mention it explicitly to not use arbitary parent clocks
in the devicetree wdt node, so add a SGRF_GATE for it.

Suggested-by: Leonidas P. Papadakos <papadakospan@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>