This adds support for the Arasan SDHCI controller, which is found on
the Xilinx Zynq 7000 and ZynqMP SoCs. This just adds very basic
PIO read/write support.

This submission is also missing the tap delay configuration, which is
required for the high speed modes on the ZynqMP, but this can be
added in a separate patch once it is clear how the interface for this
feature should look like.

The driver skeleton was provided by Michael, most of the actual driver
porting work was done by Thomas and some coding style fixes and write
support bug fixes added by Lucas.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Thomas Haemmerle <thomas.haemmerle@wolfvision.net>
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>