clk: shmobile: Add R-Car Gen2 clocks support

The R-Car Gen2 SoCs (R8A7790 and R8A7791) have several clocks that are
too custom to be supported in a generic driver. Those clocks can be
divided in two categories:

- Fixed rate clocks with multiplier and divisor set according to boot
  mode configuration

- Custom divider clocks with SoC-specific divider values

This driver supports both.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>