ARM: dts: am57xx-idk: add support for DDR mode

MMC2 in am571x and am572x can support DDR mode. Add IODelay
values for DDR mode in order to enable DDR mode support.

IODelay values added here are for revision 2.0 of AM572x silicon,
so DDR mode in the previous versions are not guaranteed to work
properly.

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