mmc: tmio, sh_mobile_sdhi: Add support for variable input clock frequency

Currently tmio_mmc assumes that the input clock frequency is fixed and
only its own clock divider can be changed.  This is not true in the
case of sh_mobile_sdhi; we can use the clock API to change it.

In tmio_mmc:
- Delegate setting of f_min from tmio to the clk_enable operation (if
  implemented), as it can be smaller than f_max / 512
- Add an optional clk_update operation called from tmio_mmc_set_clock()
  that updates the input clock frequency
- Rename tmio_mmc_clk_update() to tmio_mmc_clk_enable(), to avoid
  confusion with the clk_update operation

In sh_mobile_sdhi:
- Make the setting of f_max conditional; it should be set through the
  max-frequency property in the device tree in future
- Set f_min based on the input clock's minimum frequency
- Implement the clk_update operation, selecting the best input clock
  frequency for the bus frequency that's wanted

sh_mobile_sdhi_clk_update() is loosely based on Kuninori Morimoto's work
in sh_mmcif.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>