board: ti: am335x: Fix scale_vcore for beaglebones

commit 06507988 ("board: am335x: Introduce scale_vcores")
updated voltages of each board based on efuse. It updated
beagle bone specific voltages under the condition board_is_bone().
But this is true only for BeagleBoneWhite. Due to which voltages
are not configured for BBB, BBW as wrong device is being probed.

So create a common function board_is_beaglebonex() which includes
am335x based beagle family. Use this for updating voltages.

Also remove extra if condition for selecting voltages which is
done later using a switch case and match usb current limit as
before the commit 06507988.

Fixes: 06507988 ("board: am335x: Introduce scale_vcores")
Reported-by: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>