mmc: sdhci-cadence: set timing mode register depending on frequency

The MMC framework in U-Boot does not support a systematic API for
timing switch like mmc_set_timing() in Linux.

U-Boot just provides a hook to change the clock frequency via
mmc_set_clock().  It is up to drivers if additional register
settings are needed.

This driver needs to set a correct timing mode into a register when
it migrates to a different speed mode.  Only increasing clock frequency
could result in setup/hold timing violation.

The timing mode should be decided by checking MMC_TIMING_* like
drivers/mmc/host/sdhci-cadence.c in Linux, but "timing" is not
supported by U-Boot for now.  Just use mmc->clock to decide the
timing mode.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>