mmc: sdhci: Test cd-gpio instead of SDHCI presence when probing

Previously to this patch, an SDHCI platform that uses a GPIO for
card detection instead of the internal SDHCI_CARD_PRESENT bit on
the presence register would fail to detect a new card.

Some drivers worked around this in various ways: esdhc-imx defines
an IO accessor to fake the presence bit being true, s3c turns on
polling (which stops the SDHCI driver from checking the bit) after
a card's inserted.  But none of this should be necessary; the real
fix is to check whether we're using a GPIO and avoid relying on
the presence bit if so, as this patch implements.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>