mmc: sdhci: add platform set_timeout hook

Currently the common code assume 0xE is the maximum timeout counter
value and use it to write into the timeout counter register.
However, it's fairly possible that some other SoCs may have different
max timeout register value. That means 0xE may be incorrect and
becomes meaningless.

It's also possible that other platforms has different timeout
calculation algorithm. To be flexible, this patch provides a .set_timeout
hook for those platforms to set the timeout on their way if they need.

Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>