sdhci: implement the Host Control 2 register (tuning sequence)

sdhci: implement the Host Control 2 register (tuning sequence)

[based on a patch from Alistair Francis <alistair.francis@xilinx.com>
 from qemu/xilinx tag xilinx-v2015.2]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <20180208164818.7961-20-f4bug@amsat.org>