ARM: dts: sbc-t54: fix model property

CM-T54 CoM can be used with various custom baseboards, other
than SB-T54 (supplied with SBC-T54 single board computer).

Update model property of SBC-T54 DT to clarify this.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>