Allow mounting the SD card with SDMMC_FREQ_HIGHSPEED (BSP-286)

[igrr]

I would like to be able to mount the SD card on ESP32-S3-EYE with host.max_freq_khz = SDMMC_FREQ_HIGHSPEED.

Currently I have to copy the SD card mounting code from the BSP component into the app just to modify this setting.

Could we consider either or both solutions:

1. Add a way to change the SD card frequency setting, either via Kconfig or a run-time parameter.
2. Always set max_freq_khz = SDMMC_FREQ_HIGHSPEED on boards where we know this works. (But still provide a way to fall back to a lower frequency, same as 1.)