disk: sdmmc_stm32: support clock bypass

Add support for the SDMMC clock bypass feature for those SoCs that have
it. This provides a SDMMC bus speed double that of `clk-div = <0>`.

Updated the `clk-div` documentation at the same time to be clearer on
how the bus clock speed is determined.

Signed-off-by: Jordan Yates <jordan@embeint.com>