ARM: dts: am57xx-idk-common: Fix input supply names

Palmas Regulator is an exception and does not follow the standard
"vin-supply" common definitions for all regulators, as a result of this,
the input supplies are not reported to regulator framework, with the
obvious result of not being appropriately mapped. Fix the same.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>