Implement CMD53 block mode support for SDHCI and AllWinner-based boards

If a custom block size requested, use it, otherwise revert to the previous logic
of using just a data size if it's less than MMC_BLOCK_SIZE, and MMC_BLOCK_SIZE otherwise.

Reviewed by: bz
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D19783