MMC headers learn about SPI

Teach the MMC/SD/SDIO system headers that some hosts use SPI mode

 - New host capabilities and status bits
    * MMC_CAP_SPI, with mmc_host_is_spi() test
    * mmc_host.use_spi_crc flag

 - SPI-specific declarations:
    * Response types, MMC_RSP_SPI_R*
    * Two SPI-only commands
    * Status bits used native to SPI:  R1_SPI_*, R2_SPI_*

 - Fix a few (unrelated) whitespace bugs in the headers.

 - Reorder a few mmc_host fields, removing several bytes of padding

None of these changes affect current code.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>