dt-bindings: mmc: sdhci-am654: Add flag to force setting of TESTCD bit

The ARASAN MMC controller on Keystone 3 class of devices needs the SDCD
line to be connected for proper functioning. Similar to the issue pointed
out in sdhci-of-arasan.c driver, commit 3794c542

 ("mmc:
sdhci-of-arasan: Set controller to test mode when no CD bit").

In cases where SDCD line is not connected, adding
"ti,fails-without-test-cd" in the DT node helps to indicate the
controller, that the SDCD line has been pulled down, using the TESTCD bit.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Link: https://lore.kernel.org/r/20220425063120.10135-2-a-govindraju@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>