arm: am57xx: Fix omap_vcores assignment for am572x-idk

Currently omap_vcores is wrongly assigned a default value of
beagle_x15_volts. Hence populating a new structure for am572x-idk
and assigning it to omap_vcores in the vcores_init function.

Fixes: c020d355 ("board: ti: am57xx: Add support for am572x idk in SPL")
Reported-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>