ARM: dts: am57xx-beagle-x15/am57xx-idk: Fix pinctrl-names

The IO lines of MMC2 in am57xx-beagle-x15/am57xx-idk is connected to
3.3v. Use "ddr_3_3v" instead of "ddr_1_8v" to indicate DDR mode
works in 3.3v on these boards.

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