mmc: sdhci_am654: Add workaround for card detect debounce timer

There is a one time delay because of a card detect debounce timer in the
controller IP. This timer runs as soon as power is applied to the module
regardless of whether a card is present or not and any writes to
SDHCI_POWER_ON will return 0 before it expires. This timeout has been
measured to be about 1 second in am654x and j721e.

Write-and-read-back in a loop on SDHCI_POWER_ON for a maximum of
1.5 seconds to make sure that the controller actually powers on.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Link: https://lore.kernel.org/r/20200825170015.32285-1-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>