arm64: dts: fsl: ls1046a: disable uarts by default

Disable the UARTs by defaultto avoid registering unused UARTs. This
effectively change the number of registered UARTS for the RDB and QDS from
4 to 2 but this seems the right thing to do.

It is especially useful when connecting other 8250 uart on PCIe for example
as the default maximum number of 8250 UARTs that can be registered is 4.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>