ARM: dts: am57xx-beagle-x15: Fix regulator populated in MMC1 dt node

For beagle x15, both the vdd and io lines are connected to the
same regulator (ldo1_reg). However vmmc_aux is populated to vdd_3v3.
Remove it.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
[tony@atomide.com: updated to apply]
Signed-off-by: Tony Lindgren <tony@atomide.com>