m5stack_core2: SDHC: SD card support added

* SD card support via sdhc-spi driver added. SPI speed is
configured to 20MHz for stability reasons. Some cards
seem to not working properly with 25HMz or even higher speeds.
* Pinmux ctrl for SPI3 pins reworked. Native cs removed
as gpio chip-selects are required.

Signed-off-by: Martin Kiepfer <mrmarteng@teleschirm.org>