mmc: sdhci: Optimize delay loops

The delay loops for reset and clock enable always take at least 1 ms
because they use mdelay(1). However they can take a lot less time e.g. less
than 50us. Use ktime and reduce the delay to 10 microseconds per loop.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>