arm64: dts: tegra210-p2180: Allow ldo2 to go down to 1.8 V

Set regulator-min-microvolt property of ldo2 to 1.8 V in
tegra210-p2180.dtsi. ldo2 is used by the sdmmc1 SDHCI controller and its
voltage needs to be adjusted down to 1.8 V to support faster signaling
modes. It appears that the comment about the SDHCI driver requesting
invalid voltages no longer applies.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>