ARM: dts: am57xx-idk-common: fix ldo1 voltage range

Voltage from LDO1 coming from the TPS659037 PMIC on
the AM57xx IDK boards can scale between 1.8V to 3.3V

Fix the min voltage for this rail in device-tree data.

1.8V operation is required for using UHS cards at higher
speed modes.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>