ARM: dts: am57xx-beagle-x15-revb1: Fix supply name used for MMC1 IO lines

The dt binding documentation of omap-hsmmc recommends using
"vmmc_aux" for IO supply lines. However
commit 0af28cc9 ("ARM: dts: am57xx-beagle-x15: Add support
for rev B1") added it as "vmmc-aux". Now that anyways it
needs to be fixed, use the generic "vqmmc" binding instead of
the deprecated "vmmc_aux".

Fixes: commit 0af28cc9 ("ARM: dts: am57xx-beagle-x15: Add
support for rev B1")

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