ARM: dts: vexpress: disable CA9 core tile sp804 timer

The motherboard sp804 timer is used, but core tile sp804 timer is not.
According to Russell King, the clock configuration is undocumented and
defaults to 32kHz which is not desireable. So mark core tile sp804 timer
as disabled.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>