MFC: r333647, r338275, r338280, r338513

• If present, take advantage of the R/W cache of eMMC revision 1.5 and later devices. These caches work akin to the ones found in HDDs/SSDs that ada(4)/da(4) also enable if existent, but likewise increase the likelihood of data loss in case of a sudden power outage etc. On the other hand, write performance is up to twice as high for e. g. 1 GiB files depending on the actual chip and transfer mode employed. For maximum data integrity, the usage of eMMC caches can be disabled via the hw.mmcsd.cache tunable.
• Get rid of the NOP mmcsd_open().
• Obtain the bus mode (MMC or SD) from the directly superordinated bus rather than reaching up to the bridge and use the cached mode in mmcsd_delete(), too.
• Use le32dec(9) for decoding EXT_CSD values where it makes sense. [1]
• Locally cache some instance variable values in mmc_discover_cards() in order to improve the code readability a bit.

Obtained from: NetBSD [1]