ARM: dts: am572x-idk: Fix mmc regulator binding

Rev 1.3A of AM572x IDK has v3_3d connected to vdd and ldo1
connected to MMC IO lines. This allows AM572x IDK to support
UHS modes since ldo1 regulator can provide 1.8v.

Fix the device tree to reflect this. This breaks support
for older versions of boards. But those are pre-production
test boards and should all be replaced by production version.

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