mmc: spi: Pull out parts shared between MMC and SPI

Abstract out the common part of private data shared between MMC
and SPI. These shall later allow to use common clock configuration
function.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>