mmc: sdhci-of-at91: add DT property to enable calibration on full reset

Add a property to keep the analog calibration cell powered.
This feature is specific to the Microchip SDHCI IP and outside
of the standard SDHCI register map.

By always keeping it on, after a full reset sequence, we make sure
that this feature is activated and not disabled.

We expose a hardware property to the DT as this feature can be used
to adapt SDHCI behavior vs. how the SDCAL SoC pin is connected
on the board.

Note that managing properly this property would reduce
power consumption on some SAMA5D2 SiP revisions.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>