clk: sunxi-ng: a64: Fix gate bit of DSI DPHY

DSI DPHY gate bit on MIPI DSI clock register is bit 15
not bit 30.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>