ARM: mvebu: set SW polling as SDHCI card detection on A388-GP

The newest revisions of A388-GP (v1.5 and higher) support only
DAT3-based card detection. Revisions < v1.5 based on GPIO detection
via I2C expander, but this solution is supposed to be deprecated on
new boards. In order to satisfy all type of hardware this commit
changes card detection to use software polling mechanism. Also a
comment is added on possible card detection options in A388-GP
DT board file.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>