fix(rpi3): initialize SD card host controller

fix(rpi3): initialize SD card host controller

Add initial configuration parameters for Rasperry Pi 3's sdhost
controller, and then configure and use those parameters.

This change allows warm reboots of UEFI on Raspberry Pi 3B+ where
existing code often fails with "unknown error". See discussion at:

https://github.com/pftf/RPi3/issues/24

The basic idea is that some initial configuration parameters
(clock rate, bus width) aren't configured into the hardware before
commands start being sent. I suspect that the particular setting
that matters is the "slow card" bit, but the initial clock setting
also seemed wrong to me.

Change-Id: I526def340def143f23f3422f1fc14c12c937ca7f
Signed-off-by: Rob Newberry <robthedude@mac.com>