clk: shmobile: r8a7779: Add clocks support

The R8A7779 SoC has several clocks that are too custom to be supported in a
generic driver. Those clocks are all fixed rate clocks with multiplier and
divisor set according to boot mode configuration.

Based on work for R-Car Gen2 SoCs by Laurent Pinchart.

Cc: devicetree@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mike Turquette <mturquette@linaro.org>