ARM: dts: am57xx-beagle-x15: mmc1: remove redundant pbias-supply property

pbias-supply is initialized in dra7.dtsi. Remove redundant initialization
of pbias-supply from MMC1 dt node in am57xx-beagle-x15.dts

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