tmio_mmc_set_clock() is full of quirks because different SoC vendors
extended this in different ways.

The original IP defines the divisor range 1/2 ... 1/512.

 bit 7 is set:    1/512
 bit 6 is set:    1/256
   ...
 bit 0 is set:    1/4
 all bits clear:  1/2

It is platform-dependent how to achieve the 1/1 clock.

I guess the TMIO-MFD variant uses the clock selector outside of this IP,
as far as I see tmio_core_mmc_clk_div() in drivers/mfd/tmio_core.c

I guess bit[7:0]=0xff is Renesas-specific extension.

Socionext (and Panasonic) uses bit 10 (CLKSEL) for 1/1.  Also, newer
versions of UniPhier SoC variants use bit 16 for 1/1024.

host->clk_update() is only used by the Renesas variants, whereas
host->set_clk_div() is only used by the TMIO-MFD variants.

To cope with this mess, promote tmio_mmc_set_clock() to a new
platform hook ->set_clock(), and melt the old two hooks into it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>