mmc: sdhci: Distinguish between base clock and maximum peripheral frequency

The sdhci controller assumes that the base clock frequency is fully supported by
the peripheral and doesn't support hardware limitations. The Linux kernel
distinguishes between base clock (max_clk) of the host controller and maximum
frequency (f_max) of the card interface. Use the same differentiation and allow
the platform to constrain the peripheral interface.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>