ARM: dts: beagle-x15: switch to new cpsw switch drv

Switch all TI AM5728 BeagleBoard-X15 boards to use new cpsw switch driver.
Those boards have 2 Ext. port wired and configured in dual_mac mode by
default. Hence, dual_mac mode has been preserved the same way between
legacy and new driver it's safe to switch drivers.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>