arm64: dts: models: use list instead of tuple for mmci interrupts

RTSM/FVP vexpress motherboard model MMCI requires dedicated interrupts
for CMD and PIO, which obviously should be expressed as a list. Current
form uses tuple and it works fine since interrupt-cells equal to 1.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>