mmc: exynos_dw_mmc: Set requested freq in get_mmc_clk() callback

By now exynos_dw_mmc driver was relying on the correct CIU clock
frequency being set on driver init. But dw_mmc core is actually trying
to change CIU clock rate dynamically, on init and in set_ios() callback,
which it's requesting via host->get_mmc_clk() callback (the name is
misleading: although it's called "get_mmc_clk()", it can actually
request both get and set operations). Implement setting the requested
rate for CIU clock in Exynos driver to achieve the correct dw_mmc core
driver operation at all times. DDR mode requires the clock to be twice
as fast (when 8 bit bus is used), so handle this too, to make DDR
function properly.

This change makes the eMMC throughput on E850-96 board twice as fast.
That's because "clock-frequency" is set to 800 MHz in E850-96 device
tree, but for DDR52 mode it should be 416 MHz (and TRM states it should
be 400 MHz for DDR50/8bit mode). The dw_mmc core is requesting 52 MHz
bus_hz for DDR52 mode, and DDR+8bit mode means it should be x2 fast, so:

    f_ciu = 2 * ciu_div * f_bus = 2 * 4 * 52e6 = 416 MHz,

where f_ciu   - freq of clock fed to DW MMC block from CMU (SDCLKIN), Hz
      f_bus   - freq of clock fed to the card (CCLKIN), Hz
      ciu_div - value of internal divider (in DW MMC block).

Another way to work that around would be overriding the
"clock-frequency" property in corresponding dts. But setting the clock
frequency dynamically as it's done here looks much neater.

This implementation follows what's done in Linux kernel dw_mmc-exynos
driver in .set_ios() callback for MMC_TIMING_MMC_DDR52 case.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

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