mmc: Add ioctl to let userspace apps send ACMDs

[John Calixto <john.calixto@modsystems.com>]

Since this patch has gone through multiple revisions now, I thought I'd
list the changes in each revision:

- v4
    - replace postsleep udelay() with usleep_range()
    - add cmd_timeout_ms field for R1B commands

- v3
    - copy data from userspace before claiming host
    - break out copy from userspace into its own function
    - verify that caller has CAP_SYS_RAWIO
    - rename ``struct sd_ioc_cmd`` to ``struct mmc_ioc_cmd`` because it
      applies generally, not just to SD
    - make struct mmc_ioc_cmd the same between 32-bit and 64-bit to
      simplify compat_ioctl()
    - export include/linux/mmc/ioctl.h when you ``make headers_install``

- v2
    - make initialization of struct declarations match kernel style
    - only allow ioctl() on whole block device, not partition
    - remove extraneous printks
    - implement compat_ioctl()
    - remove version field from ``struct sd_ioc_cmd``

John