ARM: at91/dt: add always-on to 1.8V regulator

As the SDHCI controller needs the 1.8V line to be always enabled for some eMMC
configurations, set the proper "regulator-always-on" property to the board DTS
files.
Note that the sdhci classical regulator definitions doesn't suit our controller
for this 1.8V purpose.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>