ARM: dts: mps2: use list instead of tuple for uart interrupts

MPS2 UART requires dedicated interrupts for RX, TX and overflow, which
obviously should be expressed as a list. Current form uses tuple and
it has worked so far because NVIC has interrupt-cells equal to 1.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>