ARM: dts: keystone: use common "ti,keystone" compatible instead of -evm

As suggested by Olof Johansson at
http://www.spinics.net/lists/arm-kernel/msg314009.html.
It be better just keeping a "ti,keystone" top-level compatible and
just using that to probe. If so we don't have to touch the file
for new boards in the future.

So use common "ti,keystone" compatible in keystone.c for all boards.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>