arm64: dts: axm56xx: disable sp804 timer device node

Since sp804 timer is not used and it will cause warning when booting:

iOF: amba_device_add() failed (-19) for /soc/amba/timer@8080220000

Let's disable sp804 timer device node to eliminate this warning.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>