x86: edison: Don't take SD card detect pin into consideration

There are two PCB designs in the wild which use the opposite
signaling for SD card detection. This makes U-Boot working
in one case and failing in the other. Quirk this out by
disconnecting SD card detection pin from the PCB by switching
it to mode 3. In the disconnected state the read value is always
the same and inverted to what we are expecting in the code.

BugLink: https://github.com/edison-fw/meta-intel-edison/issues/136


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>