clk: vc5: Add bindings for IDT VersaClock 5P49V6901

IDT VersaClock 6 5P49V6901 has 4 clock outputs, 4 fractional dividers.
Input clock source can be taken from either external crystal or from
external reference clock.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Alexey Firago <alexey_firago@mentor.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: linux-renesas-soc@vger.kernel.org
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>