ARM: 7726/1: mmc: mmci: Add card_busy function to improve UHS card support

To verify a signal voltage switch at initialization of UHS cards the
.card_busy callback is used. For some of the ST-variants, card busy
detection on the DAT0 pin is supported.

We extend the variant struct with a busy_detect flag to indicate
support for it. A corresponding busy detect function, which polls the
busy status bit, is then set to the .card_busy callback.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>