ARM: dts: am57xx-idk: Move common MMC/SD properties to common file

AM572x IDK and AM571x IDK boards have equivalent
design of how SD card and eMMC are connected.

The two EVMs mainly differ in IOdelay configuration
needed (because of difference in SoC used).

Move the common properties to am57xx-idk-common.dtsi
file which is common for both EVMs.

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