[rpi] Add SDHOST device driver for Raspberry Pi

SDHOST is another SD controller that is present on Raspberry Pi (the
other one is SDHC and handled by bcm2835_sdhci driver). Both
controllers are capable of providing interface to SD card, actual
configuration can be set in dtb file. At the moment custom DTBs for
RPi/RPi2 have sdhost node disabled. On RPi3 sdhost is disabled in
snapshot images by applying mmc.dtbo overlay. To enalbe both devices
user has to edit config.txt on FAT partition and remove or comment
"dtoverlay=mmc" line.

When no overlay applied on RPi3 SDHOST controls SD card and SDHC
interface can be used for SDIO. mmc.dtbo overlay disables SDHOST node
and switches SD card over to SDHC. Likewise sdhost.dtbo overlay (not
currently included in snapshot image, but can be obtained from firmare
repo[1]) disabled SDHC node and switch SD card over to SDHOST.

[1] https://github.com/raspberrypi/firmware/tree/master/boot/overlays

Submitted by: Klaus P. Ohrhallinger <k@7he.at>
Differential Revision: https://reviews.freebsd.org/D14168