clk: vc5: Add support for the input frequency doubler

The VersaClock 6 has an input frequency doubler between the input
clock mux and the predivider. Add new capability flag and support
for this frequency doubler block into the driver.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Alexey Firago <alexey_firago@mentor.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: linux-renesas-soc@vger.kernel.org
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
on Salvator-XS with the display LVDS output.
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>